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.

doc/src/sgml/sepgsql.sgml

@@ -175,6 +175,9 @@ $ for DBNAME in template0 template1 postgres; do
    <command>make</command> with a Makefile supplied by SELinux.
    You will need to locate the appropriate
    Makefile on your system; the path shown below is only an example.
+   (This Makefile is usually supplied by the
+   <filename>selinux-policy-devel</filename> or
+   <filename>selinux-policy</filename> RPM.)
    Once built, install this policy package using the
    <command>semodule</command> command, which loads supplied policy packages
    into the kernel.  If the package is correctly installed,