config: add/fix include guards where missing/broken

Signed-off-by: Mike Gelfand <mikedld@mikedld.com>

config/config-backends.h

@@ -23,6 +23,9 @@
  * Author: Daniel Stone <daniel@fooishbar.org>
  */
 
+#ifndef XSERVER_CONFIG_BACKENDS_H
+#define XSERVER_CONFIG_BACKENDS_H
+
 #ifdef HAVE_DIX_CONFIG_H
 #include <dix-config.h>
 #endif
@@ -44,3 +47,5 @@ void config_hal_fini(void);
 int config_wscons_init(void);
 void config_wscons_fini(void);
 #endif
+
+#endif /* XSERVER_CONFIG_BACKENDS_H */