pulsecore: Fix issue with circuilar definitions.

pulsecore: Fix issue with circuilar definitions.

Without this fix, errors about previous definitions are generated in
numerous locations.

Example of error:

  CC     libpulsecore_1.98_la-auth-cookie.lo
In file included from ../../src/pulsecore/source.h:46:0,
                 from ../../src/pulsecore/sink.h:40,
                 from ../../src/pulsecore/core.h:50,
                 from ../../src/pulsecore/shared.h:25,
                 from ../../src/pulsecore/auth-cookie.c:33:
../../src/pulsecore/device-port.h:40:24: error: redefinition of typedef
'pa_core'
../../src/pulsecore/core.h:29:24: note: previous declaration of
'pa_core' was here
make[3]: *** [libpulsecore_1.98_la-auth-cookie.lo] Error 1

Overall it would be nicer if we could avoid this kind of fix, but it
would require further reorganisation that I'm not prepared to undertake
right now.