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 <[e-mail]>
To: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/2033/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/mm/init.c

···
 void __init paging_init(void)
 {
 	unsigned long max_zone_pfns[MAX_NR_ZONES];
-	unsigned long lastpfn;
+	unsigned long lastpfn __maybe_unused;
 
 	pagetable_init();