meson: Build qt plugin in C++11 mode explicitly

This works implicitly most of the time, but we need to set it
explicitly for building with Android.
14 jobs for use-cpp_std-c++11 in 74 minutes and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #244638
gst indent

00:00:33

passed #244637
manifest

00:00:33

 
  Build
passed #244639
build fedora x86_64

00:06:37

passed #244640
build nodebug fedora x86_64

00:06:21

 
  Test
passed #244641
check fedora

00:07:27

passed #244642
integration testsuites fedora

00:26:37

 
  Full Builds
passed #244644
build cerbero android universal

00:20:19

passed #244645
build cerbero cross win32

00:10:20

passed #244646
build cerbero cross win64

00:04:34

passed #244643
build cerbero fedora x86_64

00:08:44

 
  Full Tests
passed #244648
valgrind bad

00:07:51

passed #244649
valgrind ges

00:05:32

passed #244647
valgrind good

00:14:05

 
  Apps
passed #244650
android universal examples

00:05:52