The dependency should be "libavif/core" instead of default subspecs

[dreampiggy]

libavif podspec specify the default subspecs to libaom, so this will cause we can not disable the libaom :((

Or the default codec will be always reset to aom, even I use the following syntax in Podfile

pod 'libavif', :subspecs => ['core', 'librav1e', 'libdav1d']