Remove pg_trace.h inclusion from c.h and add it to the .c files that need it.

Remove pg_trace.h inclusion from c.h and add it to the .c files that need it.
Only needed in 8.3 because it's already this way in HEAD, and older branches
did not support DTrace.  This allows external modules to compile on Linux
machines where SystemTap support was recently added, when the required
SystemTap headers are not present on the build machine.

Approach suggested by Tom, after a RPM build trouble report by Devrim Gunduz.