livepatch: remove extern specifier from header files

Storage-class specifier 'extern' is redundant in front of the function
declaration. According to the C specification it has the same meaning as
if not present at all. So remove it.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>