gunixmounts: Fix compilation on BSD

Commit afa82ae805 introduced a compilation
regression on BSD systems that use the sysctl(3) interface; we need to
declare the buffer len in _g_get_unix_mount_points()
BZ #659528

gio/gunixmounts.c

@@ -1023,6 +1023,7 @@ _g_get_unix_mount_points (void)
   GList *return_list;
 #ifdef HAVE_SYS_SYSCTL_H
   int usermnt = 0;
+  size_t len = sizeof(usermnt);
   struct stat sb;
 #endif