Revert broken change to pgevent.c

pgevent doesn't include the global PostgreSQL headers, for a reason,
and therefor cannot rely on defines in it...

src/bin/pgevent/pgevent.c

@@ -26,7 +26,7 @@ HANDLE		g_module = NULL;	/* hModule of DLL */
  * The maximum length of a registry key is 255 characters.
  * http://msdn.microsoft.com/en-us/library/ms724872(v=vs.85).aspx
  */
-char		event_source[256] = DEFAULT_EVENT_SOURCE;
+char		event_source[256] = "PostgreSQL";
 
 /* Prototypes */
 HRESULT		DllInstall(BOOL bInstall, LPCWSTR pszCmdLine);