vt: vt_kern.h, remove extern from functions

Unify the declarations of functions in vt_kern.h: some are with extern,
some are not. Remove extern from the former as it is not needed for
functions.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219073951.16151-7-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>