Revert "build-sys: Remove the public API stuff from libpulsecommon."

Revert "build-sys: Remove the public API stuff from libpulsecommon."

This reverts commit 24ff7196753a3273bac34e87bdcf42384f974d45.

If these files aren't compiled in both libpulse and
libpulsecommon, some things will try use non-public
functions from libpulse. Therefore those internal functions
have to be included directly in libpulsecommon.

This problem appears to be only visible with --as-needed,
which is why the problem wasn't noticed immediately. The
problem has existed also earlier, and it was fixed by
Maarten Bosmans in commit 2de2c735. The commit that is
now reverted basically reverted Maarten's commit (I didn't
know that when I wrote the bad patch).