Avoid macro-generating a trailing comma in an enum.

gcc objects to this in -pedantic mode, which means other compilers are
technically entitled to object too if they like. Usually I try to
avoid it by putting a dummy value at the end of the enum, but I forgot
in this case.

(And I didn't notice, because _my_ local builds run without -pedantic,
on the grounds that configure.ac autodetects that my system's GTK
headers are not pedantic-clean. Oh well.)