Add printf attribute to vprintf() functions

This allows GCC to check vprintf() format, and makes
`-Wsuggest-attribute=format` happy.

tagmanager/mio/mio-file.c

@@ -92,6 +92,7 @@ file_puts (MIO          *mio,
   return fputs (s, mio->impl.file.fp);
 }
 
+G_GNUC_PRINTF (2, 0)
 static gint
 file_vprintf (MIO         *mio,
               const gchar *format,

tagmanager/mio/mio-memory.c

@@ -241,6 +241,7 @@ mem_puts (MIO          *mio,
   return rv;
 }
 
+G_GNUC_PRINTF (2, 0)
 static gint
 mem_vprintf (MIO         *mio,
              const gchar *format,

tagmanager/mio/mio.h

@@ -145,7 +145,7 @@ struct _MIO {
                          const gchar *s);
   gint    (*v_vprintf)  (MIO         *mio,
                          const gchar *format,
-                         va_list      ap);
+                         va_list      ap) G_GNUC_PRINTF (2, 0);
   void    (*v_clearerr) (MIO *mio);
   gint    (*v_eof)      (MIO *mio);
   gint    (*v_error)    (MIO *mio);
@@ -198,7 +198,7 @@ gint        mio_puts                (MIO         *mio,
 
 gint        mio_vprintf             (MIO         *mio,
                                      const gchar *format,
-                                     va_list      ap);
+                                     va_list      ap) G_GNUC_PRINTF (2, 0);
 gint        mio_printf              (MIO         *mio,
                                      const gchar *format,
                                      ...) G_GNUC_PRINTF (2, 3);