Add VALIDATION to BCLog::LogFlags

This flag is for logging from within CValidationInterface (see #12994).
A separate flag is desirable as the logging can be noisy and thus may
need to be disabled without affecting other logging.

src/logging.cpp

@@ -162,6 +162,7 @@ const CLogCategoryDesc LogCategories[] =
     {BCLog::COINDB, "coindb"},
     {BCLog::QT, "qt"},
     {BCLog::LEVELDB, "leveldb"},
+    {BCLog::VALIDATION, "validation"},
     {BCLog::ALL, "1"},
     {BCLog::ALL, "all"},
 };