MIPS: loongson: common: init: Add a missing include

Add a missing include to eliminate the following sparse warnings:
warning: symbol 'prom_init' was not declared. Should it be static?
warning: symbol 'prom_free_prom_memory' was not declared. Should it be static?
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8531/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/loongson/common/init.c

@@ -9,6 +9,7 @@
  */
 
 #include <linux/bootmem.h>
+#include <asm/bootinfo.h>
 #include <asm/smp-ops.h>
 
 #include <loongson.h>