Fixing parallel make of libpq

Hi hackers,

I noticed to my annoyance that 'make -j4 -C src/interfaces/libpq'
doesn't work in a clean checkout, because it can't find
libpgcommon_shlib and libpgport_shlib. It looks like that's because the
submake-libpgport dependency is declared on the all-lib target, not on
the shlib itself. Moving it to SHLIB_PREREQS instead fixes it, patch
for which is attached.

- ilmari
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