mtdutils: add include to fix implicit function warning

Including unistd.h fixes implicit function warning for "read" and "close"
in mtdutils/mounts.c

Change-Id: I72c3a52de035600cdd794ed5e2d4cae0da26f428
diff --git a/mtdutils/mounts.c b/mtdutils/mounts.c
index c90fc8a..cd3738a 100644
--- a/mtdutils/mounts.c
+++ b/mtdutils/mounts.c
@@ -19,6 +19,7 @@
 #include <string.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <unistd.h>
 #include <sys/mount.h>
 
 #include "mounts.h"