libheif: force off gdk-pixbuf

[fxcoudert]

libheif does not depend on gdk-pixbuf (even indirectly), and the loader is not built on previous macOS versions. But with recent cmake, it became a hard error:

Package gdk-pixbuf-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gdk-pixbuf-2.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gdk-pixbuf-2.0' found
CMake Error at gdk-pixbuf/CMakeLists.txt:19 (install):
  install TARGETS given no LIBRARY DESTINATION for module target
  "pixbufloader-heif".

Seen in #182758 and https://stackoverflow.com/questions/78870274/error-when-installing-libheif-with-homebrew

We can fall back to the previous situation by forcing it off.