MIPS: Include linux/initrd.h for free_initrd_mem()

arch/mips/mm/init.c provides our implementation of free_initrd_mem(),
but doesn't include the linux/initrd.h header which declares them. This
leads to a warning from sparse:

  arch/mips/mm/init.c:501:6: warning: symbol 'free_initrd_mem' was not
    declared. Should it be static?

Fix this by including linux/initrd.h to get the declaration of
free_initrd_mem().
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: trivial@kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17172/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/init.c

@@ -31,6 +31,7 @@
 #include <linux/gfp.h>
 #include <linux/kcore.h>
 #include <linux/export.h>
+#include <linux/initrd.h>
 
 #include <asm/asm-offsets.h>
 #include <asm/bootinfo.h>