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.

tests/test_opus_encode.c

@@ -38,6 +38,9 @@
 #include <time.h>
 #if (!defined WIN32 && !defined _WIN32) || defined(__MINGW32__)
 #include <unistd.h>
+#else
+#include <process.h>
+#define getpid _getpid
 #endif
 #include "opus_multistream.h"
 #include "opus.h"