gstgl: Fix build when Meson >= 0.58.0rc1

"implicit_include_directories: false" now also means that current build
directory is not added to include paths by default any more. We have to
add it manually because we have some custom_target() that generate
headers in current build directory.

See https://github.com/mesonbuild/meson/issues/8700.

Part-of: <gstreamer/gst-plugins-base!1125>
36 jobs for !1125 with fix-meson-0-58 in 26 minutes and 17 seconds (queued for 2 seconds)
detached
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #9234743

00:00:40

passed android docker #9234742

00:00:42

passed cerbero fedora amd64 docker #9234741

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passed fedora amd64 docker #9234740

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passed gst-indent amd64 docker #9234744

00:00:41

passed windows amd64 docker #9234745
windows shell 1809

00:00:08

 
  Preparation
passed check allow-edit #9234748

00:00:37

passed gst indent #9234747

00:00:32

passed manifest #9234746

00:00:30

 
  Build
passed build cerbero cross-android universal #9234757
gstreamer

00:08:25

passed build cerbero cross-ios universal #9234761
gst-ios-14.3

00:21:23

passed build cerbero cross win32 #9234758
gstreamer

00:08:48

passed build cerbero cross win64 #9234759
gstreamer

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passed build cerbero fedora x86_64 #9234756
gstreamer

00:04:18

passed build cerbero macos x86_64 #9234760
gst-macos-11.1

00:13:12

passed build cerbero msvc x86_64 #9234762
docker windows gstreamer-windows 1809

00:18:16

passed build clang fedora x86_64 #9234752

00:04:16

manual build msys2 #9234755
docker windows 1809 allowed to fail manual
passed build nodebug fedora x86_64 #9234749

00:03:35

passed build static fedora x86_64 #9234750

00:04:17

passed build static nodebug fedora x86_64 #9234751

00:05:13

passed build vs2017 amd64 #9234753
docker windows 1809

00:18:39

passed build vs2017 x86 #9234754
docker windows 1809

00:18:54

 
  Test
passed check fedora #9234763
gstreamer

00:08:04

passed integration testsuites fedora 1/4 #9234764
gstreamer

00:07:20

passed integration testsuites fedora 2/4 #9234765
gstreamer

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passed integration testsuites fedora 3/4 #9234766
gstreamer

00:08:29

passed integration testsuites fedora 4/4 #9234767
gstreamer

00:07:34

passed valgrind bad #9234771
gstreamer

00:06:58

passed valgrind base #9234768
gstreamer

00:09:24

passed valgrind ges #9234772
gstreamer

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passed valgrind good #9234769
gstreamer

00:10:46

passed valgrind ugly #9234770
gstreamer

00:05:23

 
  Integrate
passed cross-android universal examples #9234773

00:10:54

passed cross-ios universal examples #9234775
gst-ios-14.3

00:03:34

passed documentation #9234774

00:09:55