glib-init: fix redundant-decls of g_mem_gc_friendly

glib-init: fix redundant-decls of g_mem_gc_friendly

Currently g_mem_gc_friendly is declared in both gmem.h and glib-init.h
files, we will have reports on each unit that include these two files.

This patch removes the redundant declaration from glib-init.h

Since g_mem_gc_friendly is related to gmem.h and was first declared in
this header which also exports it via glib.h, then declare it in gmem.h

Other files already include gmem.h: garray.c and gslice.c, no need to
change anything.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
https://bugzilla.gnome.org/show_bug.cgi?id=710345