ANDROID: mm: Export do_munmap

The 0-day build bot reports the following build error, seen if SDCARD_FS
is built as module.

ERROR: "do_munmap" undefined!

Fixes: 84a1b7d3d312 ("Included sdcardfs source code for kernel 3.0")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Guenter Roeck <groeck@chromium.org>
diff --git a/mm/mmap.c b/mm/mmap.c
index 590df38..143d62f 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2672,6 +2672,7 @@
 
 	return 0;
 }
+EXPORT_SYMBOL(do_munmap);
 
 int vm_munmap(unsigned long start, size_t len)
 {