Rename argument to avoid preprocessor problems

Rename argument to avoid preprocessor problems

Before this change, we were getting the following error with GUB:

/home/gub/gub/target/mingw/src/
lilypond-git.sv.gnu.org--lilypond.git-release-unstable/
lily/include/paper-column.hh:62:
error: expected identifier before ',' token
/home/gub/gub/target/mingw/src/
lilypond-git.sv.gnu.org--lilypond.git-release-unstable/
lily/include/paper-column.hh:62:
error: two or more data types in declaration of 'parameter'

David Kastrup suspects that the library/compiler used for bootstrapping
has a macro or reserved word defined for "interface" - changing the name
makes the problem go away.