Rename the remaining main.{h,cpp} to validation.{h,cpp}

src/test/sigopcount_tests.cpp

@@ -2,7 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include "main.h"
+#include "validation.h"
 #include "pubkey.h"
 #include "key.h"
 #include "script/script.h"