variable_watch: Add missing string enumeration entry (#14188)

In commit 9a5b4eba (All variable accesses should produce watch
callbacks, 2007-05-17) we added a new enumeration value but did not
update the list of strings matching them.  Add the missing entry.
Without this, variable_watch prints REMOVED_ACCESS instead of
MODIFIED_ACCESS when set the value of the variable.

Source/cmVariableWatch.cxx

@@ -15,6 +15,7 @@ static const char* const cmVariableWatchAccessStrings[] =
 {
     "READ_ACCESS",
     "UNKNOWN_READ_ACCESS",
+    "UNKNOWN_DEFINED_ACCESS",
     "ALLOWED_UNKNOWN_READ_ACCESS",
     "MODIFIED_ACCESS",
     "REMOVED_ACCESS",