add grub_qsort_strcmp to use when sorting array of strings

Compare function used in qsort gets arguments by reference, so strcmp
cannot be used directly - it expects pointer to char, but gets pointer
to pointer to char.

Introduce new helper grub_qsort_strcmp and use it in grub-install.
This helper is going to be used in a couple more places as well so
add it to global file, not in grub-install.c.

util/misc.c

@@ -256,3 +256,11 @@ void
 grub_register_exported_symbols (void)
 {
 }
+
+/* Used in comparison of arrays of strings with qsort */
+int
+grub_qsort_strcmp (const void *p1, const void *p2)
+{
+  return strcmp(*(char **)p1, *(char **)p2);
+}
+