meson: qmlgl plugin iOS definitions

Tested with cross-ios-arm64 and cross-ios-x86, since those two are the
only archs shipped with the official Qt binaries.
8 jobs from fix-qt-ios-meson-build in 11 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #95961
gst indent

00:00:14

passed #95960
manifest

00:00:17

 
  Build
passed #95962
build fedora x86_64

00:01:52

 
  Test
passed #95963
check fedora

00:02:59

 
  Full Builds
passed #95964
build android arm64 api28

00:02:47

passed #95966
build cerbero cross win32

00:05:30

passed #95967
build cerbero cross win64

00:05:13

passed #95965
build cerbero fedora x86_64

00:06:17