pgsql: Doc: explain where to find Makefile used to buildsepgsql-regtes

Doc: explain where to find Makefile used to build sepgsql-regtest.pp.

At least on Fedora and RHEL, it's not in the same RPM that's needed
for building sepgsql itself.  Today is the second or third time I've
had to rediscover how to install that, so let's document it this time.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/860c095fd548cd25586e4273e9b489082b4ffa13

Modified Files
--------------
doc/src/sgml/sepgsql.sgml | 3 +++
1 file changed, 3 insertions(+)