glib-unix: Fix build of safe_fdwalk() on Solaris

The refactoring done by commit 168fd4f2b3
lost the definition of the open_max variable used in the Solaris ifdefs.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

glib/glib-unix.c

@@ -699,6 +699,7 @@ safe_fdwalk (int (*cb)(void *data, int fd), void *data)
  * fcntl(fd, F_PREVFD)
  * - return highest allocated file descriptor < fd.
  */
+  gint open_max;
   gint fd;
   gint res = 0;