membuf: Include stdbool

This uses a bool type so include the required header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/membuf.h b/include/membuf.h
index 17616d5..636ed70 100644
--- a/include/membuf.h
+++ b/include/membuf.h
@@ -9,6 +9,8 @@
 #ifndef _membuf_H
 #define _membuf_H
 
+#include <stdbool.h>
+
 /**
  * @struct membuf: holds the state of a membuff - it is used for input and
  * output buffers. The buffer extends from @start to (@start + @size - 1).