fix(cmake): link missing static deps for GStreamer opengl plugin

[ttr-007]

Summary

• The statically-linked gstopengl GStreamer plugin has no pkg-config file, so FindGStreamer.cmake falls back to a bare find_library() call that only adds the plugin archive itself, with no transitive dependencies.
• Internally gstopengl links against graphene, libpng, libjpeg, and gstreamer-controller-1.0 — none of which were being linked in, causing undefined-symbol link errors whenever the opengl plugin is enabled (e.g. QGC_ENABLE_HERELINK=ON on Android, currently the only way to build against Qt 6.6.3 / minSdk 25 for Android 7.1–8.x support, per #13371).
• This adds the four missing static libraries to GStreamer::Plugins whenever opengl is in GSTREAMER_PLUGINS.

How this was found

Building QGroundControl for Android arm64-v8a on Stable_V5.0 with Qt 6.6.3 (-DQGC_ENABLE_HERELINK=ON, matching .github/workflows/android-linux-qt6.6.3.yml) compiled cleanly (~400+ translation units) but failed at the final link step. The four missing symbols surfaced one at a time as each was fixed and relinked:

Missing symbols	From	Library
graphene_matrix_*, graphene_plane_*	gstgltransformation.c	libgraphene-1.0.a
png_read_info, png_get_IHDR, etc.	gstgloverlay.c	libpng16.a
jpeg_read_header, jpeg_start_decompress, etc.	gstgloverlay.c	libjpeg.a
gst_proxy_control_binding_new	gstglsinkbin.c / gstglvideomixer.c	libgstcontroller-1.0.a

Note: GStreamer does ship graphene-gobject-1.0.pc etc., but the embedded prefix= in those .pc files is hard-coded to the original Cerbero build machine's path, so pkg_check_modules can't resolve them — hence using find_library() directly here, consistent with the existing fallback pattern in this file.

After applying this fix, the build linked and packaged successfully, producing an installable minSdkVersion=25 APK verified via aapt dump badging.

Test plan

• Configured and built QGroundControl for Android arm64-v8a, Qt 6.6.3, Stable_V5.0, with QGC_ENABLE_HERELINK=ON
• Build linked and packaged successfully (previously failed with undefined-symbol errors)
• Verified output APK with aapt dump badging (sdkVersion:'25', native-code: 'arm64-v8a')
• Not tested on Linux/macOS CI runners — the underlying issue should reproduce there too since it's not platform-specific, but I only verified on Windows

🤖 Generated with Claude Code

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[ttr-007]

Closing — keeping this fix on my fork only for now, not intending to merge upstream at this time.