MIPS: Fix GCC-4.6 'set but not used' warning in arch/mips/mm/init.c

Under some combinations of CONFIG_*, lastpfn in page_is_ram is 'set
but not used'.  Mark it as __maybe_unused to quiet the warning/error.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2033/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/init.c

@@ -324,7 +324,7 @@ int page_is_ram(unsigned long pagenr)
 void __init paging_init(void)
 {
 	unsigned long max_zone_pfns[MAX_NR_ZONES];
-	unsigned long lastpfn;
+	unsigned long lastpfn __maybe_unused;
 
 	pagetable_init();