opus

Include process.h for _getpid on windows.

Previously the test programmes happened to link because _getpid
returns an int, which is the default prototype. Instead, include
the appropriate header to supply a real declaration, fixing the
associated warning.

I moved the getpid->_getpid define to the site of the header
inclusion to make clear what's going on, rather than have it
floating in win32/config.h.