build-sys: Fix building with --as-needed

build-sys: Fix building with --as-needed

pulsecore/core-format.c was recently added to libpulsecommon, and
core-format.c depends on functions in libpulse, which libpulsecommon
doesn't link to. That broke building with --as-needed. This patch adds
pulse/format.c to libpulsecommon, so that core-format.c doesn't need
to depend on libpulse any more. format.c pulls in also the dependency
to json-c.

Reported-By: Jan Steffens <jan.steffens@gmail.com>