Add missed "make clean" logic in src/interfaces/libpq/Makefile.

Add missed "make clean" logic in src/interfaces/libpq/Makefile.

Commit 818fd4a67 missed cleaning up the symlinks it added for various .c
files imported from src/port and src/common.  Neatnik-ishly make the
file lists in the "clean" target look exactly like the earlier lists of
what to symlink in.