vt(4): Enclose vt_mouse_paste() prototype inside #ifndef SC_NO_CUTPASTE/#endif

This fixes the build with sparc64 LINT for instance.

Reported by:	bz@
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271506 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f