livepatch: make klp_mutex proper part of API

klp_mutex is shared between core.c and transition.c, and as such would
rather be properly located in a header so that we don't have to play
'extern' games from .c sources.

This also silences sparse warning (wrongly) suggesting that klp_mutex
should be defined static.

Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>