env_mmc: fix compile warning

hexport would complain implicit declaration, if we don't add the
include file.

env_mmc.c: In function 'saveenv':
env_mmc.c:109: warning: implicit declaration of function 'hexport'

Signed-off-by: Lei Wen <leiwen@marvell.com>

common/env_mmc.c

@@ -29,6 +29,7 @@
 #include <linux/stddef.h>
 #include <malloc.h>
 #include <mmc.h>
+#include <search.h>
 #include <errno.h>
 
 /* references to names in env_common.c */