pgsql: Reduce presence of syscache.h in src/include/

Reduce presence of syscache.h in src/include/

ee642cccc43c has added syscache.h in inval.h and objectaddress.h,
enlarging by a lot the footprint of this header, particularly via
objectaddress.h. A change in syscache.h would cause a lot more files to
be recompiled.

This commit reduces the presence of syscache.h by switching to a direct
use of syscache_ids.h in inval.h and objectaddress.h, where the enum
SysCacheIdentifier is defined. genbki.pl gains an #ifndef block for
this header, so as its inclusion is more controlled.

Reported-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/vlcexdcimsmvu3aplt2yxpfndkgtuvjsrms2fdl46rbw3k2kug@drspkoxlaije

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e0fa5bd146564d9d05cac15bdcba65b7860d2b91

Modified Files
--------------
src/backend/catalog/genbki.pl | 10 ++++++++--
src/include/catalog/objectaddress.h | 2 +-
src/include/utils/inval.h | 2 +-
3 files changed, 10 insertions(+), 4 deletions(-)