Add __attribute__((format(printf, a, b))) to printf like functions.

Fix potential string format bug.

Change-Id: Ie05aac53b2c45a48bd68e340b76ccb21edfd28b7
diff --git a/common.h b/common.h
index ff577c2..1182d77 100644
--- a/common.h
+++ b/common.h
@@ -31,7 +31,7 @@
 // Write a message to the on-screen log shown with Alt-L (also to stderr).
 // The screen is small, and users may need to report these messages to support,
 // so keep the output short and not too cryptic.
-void ui_print(const char *fmt, ...);
+void ui_print(const char *fmt, ...) __attribute__((format(printf, 1, 2)));
 
 // Display some header text followed by a menu of items, which appears
 // at the top of the screen (in place of any scrolling ui_print()