meson.build: fix detection of gio/gdesktopappinfo.h

Header file gio/gdesktopappinfo.h is currently never found by Meson. Any GTK4 build shows an error like this.

This bug was introduced by commit 13b94dab which fixed the build for MacOS but caused the header file to not be detected by Linux anymore.

One possible fix would be to replace the string gio/gdesktopappinfo.h with gio-unix-2.0/gio/gdesktopappinfo.h, but being a unix only header we preferred moving the check to an appropiate place where other unix specific handling is done.