sequencer: make sign_off_header a file local symbol

sequencer: make sign_off_header a file local symbol

Commit d0aaa46fd3 ("commit: move empty message checks to libgit",
2017-11-10) removes the last use of 'sign_off_header' outside of
the "sequencer.c" source file. Remove the extern declaration from
the header file and mark the definition of the symbol with the
static keyword.

Signed-off-by: Ramsay Jones <redacted>
Signed-off-by: Junio C Hamano <redacted>