Harden be-gssapi-common.h for headerscheck

Surround the contents with a test that the feature is enabled by
configure, to silence header checking tools on systems without GSSAPI
installed.

Backpatch to 12, where the file appeared.

Discussion: https://postgr.es/m/202111161709.u3pbx5lxdimt@alvherre.pgsql

Author: alvherre

src/include/libpq/be-gssapi-common.h

@@ -14,6 +14,8 @@
 #ifndef BE_GSSAPI_COMMON_H
 #define BE_GSSAPI_COMMON_H
 
+#ifdef ENABLE_GSS
+
 #if defined(HAVE_GSSAPI_H)
 #include <gssapi.h>
 #else
@@ -23,4 +25,6 @@
 extern void pg_GSS_error(const char *errmsg,
 						 OM_uint32 maj_stat, OM_uint32 min_stat);
 
+#endif							/* ENABLE_GSS */
+
 #endif							/* BE_GSSAPI_COMMON_H */