scripts/get_maintainer.pl: using --separator implies --nomultiline

If a person sets a separator, it's only used if --nomultiline is set.
Don't make the command line also include --nomultiline in that case.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>