Remove dead "-framework AGL" link directive (macOS 10.14+ and all Apple Silicon)

[geekrebel]

The APPLE branch of CMakeLists.txt links the AGL framework:

if(APPLE)
  ...
  target_link_libraries(openshot-audio PRIVATE
    ...
    "-framework IOKit"
    "-framework AGL"
    "-framework AudioToolbox"
    ...
  )

AGL (Apple Graphics Library) was deprecated long ago and has been
absent from the macOS SDK since macOS 10.14 Mojave (2018), so on any
reasonably modern Xcode or Command Line Tools the link step fails with:

ld: framework 'AGL' not found
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [libopenshot-audio.0.6.0.dylib] Error 1

That includes every Apple Silicon machine, since Apple Silicon only supports
macOS 11+, so the project cannot currently be built from source on any
M1/M2/M3 Mac without patching this file.

AGL is not actually used by libopenshot-audio at runtime: it appears to
have been inherited from an old JUCE integration example, and modern macOS
acquires its OpenGL context through Cocoa (NSOpenGLContext) rather than
AGL. Removing the link directive has no behavioural effect beyond letting
the library link successfully.

Verification

Verified on an Apple Silicon (arm64) Mac:

• OS: macOS 15
• Toolchain: AppleClang 17.0.0, CMake 4.0.3, Command Line Tools installed
• Steps: cmake -B build -S . && cmake --build build -j
• Result with this patch: openshot-audio.dylib and
  openshot-audio-demo both build and link successfully. Without the
  patch, link fails at openshot-audio.dylib with the error above.

Scope

Intentionally minimal: one line removed, no behavioural changes, no other
framework list edits. Happy to follow up with a small macOS build section
in INSTALL.md in a separate PR if useful, since the current instructions
are headed "Linux Build Instructions" and don't document the macOS path.

Context

This is my first PR to libopenshot-audio. It is part of a small series of
Apple Silicon contributor fixes: openshot-qt#6003,
#6004,
#6005, and
#6006 cover the
packaging-side changes needed before an arm64 DMG can be produced; this PR
is the first of the corresponding libopenshot-side fixes and also resolves
the root cause of libopenshot#1030,
whose reporter hit the follow-on "cannot find OpenShotAudio" CMake error
because libopenshot-audio cannot be built on their machine in the first place.