Properly install dynloader.h on MSVC builds

This will enable PL/Java to be cleanly compiled, as dynloader.h is a
requirement.

Report by Chapman Flack

Patch by Michael Paquier

Backpatch through 9.1

src/backend/utils/fmgr/dfmgr.c

@@ -16,11 +16,7 @@
 
 #include <sys/stat.h>
 
-#ifndef WIN32_ONLY_COMPILER
 #include "dynloader.h"
-#else
-#include "port/dynloader/win32.h"
-#endif
 #include "lib/stringinfo.h"
 #include "miscadmin.h"
 #include "storage/shmem.h"