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
------
V4_6_STABLE

Details
-------
https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=1793db95faa8340d32d7c2a7577a0e33895d7e3d

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