pgpool: Fix point less warning in query cache invalidation.

Fix point less warning in query cache invalidation.

When memcached support is disabled, query cache invalidation by query
emitted point less warning. This makes 006.memcached regression test
failed.

PGPOOL SET CACHE DELETE '/*FORCE QUERY CACHE*/SELECT 1;';
WARNING: failed to delete query cache on memcached, memcached support is not enabled

Backpatch-through: v4.6

Branch
------
master

Details
-------
https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=e22467bf3d8a5620ca81567d30c2095def8fb016

Modified Files
--------------
src/query_cache/pool_memqcache.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)