Mk/Uses/gnome.mk: fix gconftool-2 --makefile-unistall-rule call time

Previously, for ports with GCONF_SCHEMAS, `gnome-post-gconf-schemas'
target would add a @postunexec rule which was invoked *after* those
files would be removed upon deinstallation, resulting in errors:

I/O warning : failed to load external entity ".../$some.schemas"
Failed to open `.../$some.schemas': No such file or directory

Change it to @preunexec so that `gconftool-2 --makefile-unistall-rule'
command would get called while those files still exist.

PR: 210969
Timeout from: gnome (~5 years)