sql parser: identifier can start with the "_" character

git-svn-id: trunk@46737 -
(cherry picked from commit 39c8d5c635d21c04fed3cacfe7cfb29a68e2818f)

packages/fcl-db/src/sql/fpsqlscanner.pp

@@ -897,7 +897,7 @@ begin
         Result := tsqlGT;
       end;
    'a'..'z',
-   'A'..'Z':
+   'A'..'Z', '_':
      Result:=DoIdentifier;
    else
      // Symbol of some sort