meson: Only ensure that moc is available on Linux

On other OSes, it's not possible to have qmake or the qt5 pkg-config
files and not have moc, and `moc` will not be in `PATH`, so this only
causes problems.
12 jobs for <span class="ref-name">qt5-dont-check-moc</span> in 49 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #206346
gst indent

00:00:28

passed #206345
manifest

00:00:35

 
  Build
passed #206347
build fedora x86_64

00:04:37

passed #206348
build nodebug fedora x86_64

00:06:51

 
  Test
passed #206349
check fedora

00:04:20

 
  Full Builds
passed #206351
build cerbero android universal

00:13:07

passed #206352
build cerbero cross win32

00:11:38

passed #206353
build cerbero cross win64

00:05:09

passed #206350
build cerbero fedora x86_64

00:06:03

 
  Full Tests
passed #206355
valgrind bad

00:07:53

passed #206354
valgrind good

00:09:40

 
  Apps
passed #206356
android universal examples

00:14:14