MXS-862: Add FindGSSAPI.cmake and missing includes

Added FindGSSAPI.cmake which allows the modules to be built only if the
libraries are found.

The log manager header was not included by the GSSAPI modules.

server/modules/authenticator/gssapi_auth_common.c

@@ -13,6 +13,7 @@
 
 #include "gssapi_auth.h"
 #include <maxscale/alloc.h>
+#include <maxscale/log_manager.h>
 
 void* gssapi_auth_alloc(void *instance)
 {