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"},
 };

src/logging.h

@@ -54,6 +54,7 @@ namespace BCLog {
         COINDB      = (1 << 18),
         QT          = (1 << 19),
         LEVELDB     = (1 << 20),
+        VALIDATION  = (1 << 21),
         ALL         = ~(uint32_t)0,
     };