Add G_GNUC_PRINTF on all functions with format strings

This allows compilation with clang without errors, even when
-Wformat-nonliteral is active (as long as there are no real cases of
non literal formatting).

https://bugzilla.gnome.org/show_bug.cgi?id=691608

glib/gmarkup.h

@@ -214,7 +214,7 @@ gchar* g_markup_escape_text (const gchar *text,
 gchar *g_markup_printf_escaped (const char *format,
 				...) G_GNUC_PRINTF (1, 2);
 gchar *g_markup_vprintf_escaped (const char *format,
-				 va_list     args);
+				 va_list     args) G_GNUC_PRINTF(1, 0);
 
 typedef enum
 {