build-aux: fix warning in the built _get_string() methods with -Wsign-compare

mm-enums-types.c: In function ‘mm_modem_lock_get_string’:
  mm-enums-types.c:165:17: warning: comparison of integer expressions of different signedness: ‘MMModemLock’ {aka ‘enum <anonymous>’} and ‘gint’ {aka ‘const int’} [-Wsign-compare]
    165 |         if (val == mm_modem_lock_values[i].value)
        |                 ^~

build-aux/mm-enums-template.c

@@ -51,7 +51,7 @@ const gchar *
     guint i;
 
     for (i = 0; @enum_name@_values[i].value_nick; i++) {
-        if (val == @enum_name@_values[i].value)
+        if ((gint)val == @enum_name@_values[i].value)
             return @enum_name@_values[i].value_nick;
     }