Add -Wsuggest-attribute=format warning, correct warnings thus generated

Add this new warning to the default build flags.  It suggests adding
a ((format)) attribute to xavsprintf_append(), so do that.

Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile

@@ -22,7 +22,8 @@ ASSUME_MASK ?= 0
 
 CPPFLAGS = -I libfdt -I . -DFDT_ASSUME_MASK=$(ASSUME_MASK)
 WARNINGS = -Wall -Wpointer-arith -Wcast-qual -Wnested-externs -Wsign-compare \
-	-Wstrict-prototypes -Wmissing-prototypes -Wredundant-decls -Wshadow
+	-Wstrict-prototypes -Wmissing-prototypes -Wredundant-decls -Wshadow \
+	-Wsuggest-attribute=format
 CFLAGS = -g -Os $(SHAREDLIB_CFLAGS) -Werror $(WARNINGS) $(EXTRA_CFLAGS)
 
 BISON = bison

meson.build

@@ -14,7 +14,8 @@ add_project_arguments(
     '-Wstrict-prototypes',
     '-Wmissing-prototypes',
     '-Wredundant-decls',
-    '-Wshadow'
+    '-Wshadow',
+    '-Wsuggest-attribute=format',
   ]),
   language: 'c'
 )

util.h

@@ -65,7 +65,7 @@ extern char *xstrndup(const char *s, size_t len);
 
 extern int PRINTF(2, 3) xasprintf(char **strp, const char *fmt, ...);
 extern int PRINTF(2, 3) xasprintf_append(char **strp, const char *fmt, ...);
-extern int xavsprintf_append(char **strp, const char *fmt, va_list ap);
+extern int PRINTF(2, 0) xavsprintf_append(char **strp, const char *fmt, va_list ap);
 extern char *join_path(const char *path, const char *name);
 
 /**