You can add a little bit of table salt to it. Put a dash in a pint, add a shot of beer, let it foam, and let the salt to dissolve. Now fill the pint. If not enough SLOWLY try adding more until you get it right. Figure out how much per pint and then dissolve that amount in some beer and pour it into the keg.
If you are lucky, the amount you need will be below the taste threshold for salt. This is one of the roles of salt in cooking - to mute bitter flavors. If you want to see how powerful it is, take a drink of straight tonic water, now take a good lick of some salt, and try the tonic water again. With enough salt, it just tastes like sugar water.