Fix compiler warnings regarding DEFAULT_PGSOCKET_DIR.

Fix compiler warnings regarding DEFAULT_PGSOCKET_DIR.

Commit 3bd6941f accidentally add "#include "pg_config_manual.h", which
should have been "#include parser/pg_config_manual.h".  This caused to
include parser/pg_config_manual.h and pg_config_manual.h, which caused
compiler warning:

../src/include/parser/pg_config_manual.h:227:0: warning: "DEFAULT_PGSOCKET_DIR" redefined [enabled by default]
 #define DEFAULT_PGSOCKET_DIR  "/tmp"

If DEFAULT_PGSOCKET_DIR value was different among these header files.

Reported by: Peng Bo.