Makefile: ignore glib-genmarshal in staging_dir

HOST_GLIB is set to the path that contains the host glib tool set and is
used when building packages using glib. The buildroot top level Makefile
sets HOST_GLIB using which to find the path where glib-genmarshal is
located.

The problem is that a cross compiled version of glib-genmarshal is also
put in the build_ARCH/staging_dir/usr/bin directory when the package
libglib2 is built. This cross compiled version will typically not run on
the host system.

Fix it by ignoring staging_dir in the which output.

Closes #5934

jacmet: fixed to work correctly if it's only found in staging_dir.