exfat: add necessary header for vmalloc

This fixes building on Linux 4.4.

Signed-off-by: HiGarfield <higarfield@126.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
diff --git a/balloc.c b/balloc.c
index 21a865e..c226d5d 100644
--- a/balloc.c
+++ b/balloc.c
@@ -12,6 +12,7 @@
 #else
 #include <linux/sched.h>
 #endif
+#include <linux/vmalloc.h>
 
 #include "exfat_raw.h"
 #include "exfat_fs.h"