Fix breakage in making pgpool_adm.

Fix breakage in making pgpool_adm.

Since commit f54efc50 "Fix compiler warnings regarding
DEFAULT_PGSOCKET_DIR.", src/sql/pgpool_adm cannot be compiled with
error:
In file included from pgpool_adm.c:35:0:
../../include/pcp/libpcp_ext.h:32:37: fatal error: parser/pg_config_manual.h: No such file or directory
 #include "parser/pg_config_manual.h"

Since pgpool_adm is a PostgreSQL extension module, it should not
include Pgpool-II's pg_config_manual.h.  Rather than fixing pgpool_adm
to allow to include pgpool's pg_config_manual.h, modify libpcp_ext.h
so that it does not include pgpool's pg_config_manual.h if PGPOOL_ADM
is defined.