lame: fix build if host has libgtk12-dev (gtk-config)

Lame contains an optional gtk12 based frame analyzer application, which
will be built if gtk12 is detected (using gtk-config). We no longer
have support for (the obsolete) gtk12 in BR, but the host might have
gtk-config, so forcible disable it.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>