gssapi: show information about disabled GSSAPI

When USE_GSSAPI=OFF, still show information about what SPNEGO is, even
though it's disabled.  This is for parity with other disabled features
like SSH and debugpool that still show details about what is disabled.

cmake/Modules/SelectGSSAPI.cmake

@@ -49,5 +49,5 @@ IF(GSS_BACKEND)
 	ENDIF()
 ELSE()
 	SET(GIT_GSSAPI 0)
-	ADD_FEATURE_INFO(SPNEGO NO "")
+	ADD_FEATURE_INFO(SPNEGO NO "SPNEGO authentication support")
 ENDIF()