*: check defined(_WIN32) instead of defined(WIN32)

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

src/client/ClientFile.cxx

@@ -28,7 +28,7 @@
 void
 Client::AllowFile(Path path_fs) const
 {
-#ifdef WIN32
+#ifdef _WIN32
 	(void)path_fs;
 
 	throw ProtocolError(ACK_ERROR_PERMISSION, "Access denied");