multimedia/aom: avoid leaking libjxl and vmaf via pkg-config

$ make stage-qa -C graphics/libavif
[...]
Error: /usr/local/lib/libavif.so.14.0.1 is linked to /usr/local/lib/libvmaf.so.1 from multimedia/vmaf but it is not declared as a dependency
Warning: you need LIB_DEPENDS+=libvmaf.so:multimedia/vmaf
Error: /usr/local/lib/libavif.so.14.0.1 is linked to /usr/local/lib/libjxl.so.0.7 from graphics/libjxl but it is not declared as a dependency
Warning: you need LIB_DEPENDS+=libjxl.so:graphics/libjxl

Reported by: Tatsuki Makino (on ports@ list), Jan Henrik Sylvester (private mail)