Silence clang errors about -Wformat-nonliteral due to missing intermediate attributes

By default, meson builds glib with -Werror=format=2, which
implies -Werror=format-nonliteral. With these flags, clang errors
out on e.g. the g_message_win32_error function, due to "format
string is not a string literal". This function takes a format
string, and passes the va_list of the arguments onwards to
g_strdup_vprintf, which is annotated with printf attributes.

When passing a string+va_list to another function, GCC doesn't warn
with -Wformat-nonliteral. Clang however does warn, unless the
functions themselves (g_message_win32_error and set_error) are decorated
with similar printf attributes (to force the same checks upon the
caller) - see
https://clang.llvm.org/docs/AttributeReference.html#format
for reference.

Adding these attributes revealed one existing mismatched format string
(fixed in the preceding commit).

gio/gregistrysettingsbackend.c

@@ -201,7 +201,7 @@ trace (const char *format,
  * equivalent function for g_warning because none of the registry errors can
  * result from programmer error (Microsoft programmers don't count), instead
  * they will mostly occur from people messing with the registry by hand. */
-static void
+static void G_GNUC_PRINTF (2, 3)
 g_message_win32_error (DWORD        result_code,
                        const gchar *format,
                       ...)

gmodule/gmodule-win32.c

@@ -39,7 +39,7 @@
 #include <sys/cygwin.h>
 #endif
 
-static void
+static void G_GNUC_PRINTF (1, 2)
 set_error (const gchar *format,
 	   ...)
 {