Don't #include <glib/gslice.h> from gmem.h

It looks like this was done just to help people port from gmem to
gslice, but nothing in this header actually requires gslice.h to be
included.

glib/ghash.c

@@ -37,6 +37,7 @@
 #include "gstrfuncs.h"
 #include "gatomic.h"
 #include "gtestutils.h"
+#include "gslice.h"
 
 
 /**