Remove a gccism

Pointed out in bug 656152.

glib/tests/checksum.c

@@ -619,9 +619,23 @@ hexval (const gchar c)
 {
   switch (c)
     {
-     case '0' ... '9':
+     case '0':
+     case '1':
+     case '2':
+     case '3':
+     case '4':
+     case '5':
+     case '6':
+     case '7':
+     case '8':
+     case '9':
        return c - '0';
-     case 'a' ... 'f':
+     case 'a':
+     case 'b':
+     case 'c':
+     case 'd':
+     case 'e':
+     case 'f':
        return 10 + c - 'a';
      default:
        return 0;