sequencer: make sign_off_header a file local symbol

Commit d0aaa46f ("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 <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -31,7 +31,7 @@
 #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
-const char sign_off_header[] = "Signed-off-by: ";
+static const char sign_off_header[] = "Signed-off-by: ";
 static const char cherry_picked_prefix[] = "(cherry picked from commit ";
 GIT_PATH_FUNC(git_path_commit_editmsg, "COMMIT_EDITMSG")

sequencer.h

@@ -83,8 +83,6 @@ int check_todo_list(void);
 int skip_unnecessary_picks(void);
 int rearrange_squash(void);
-extern const char sign_off_header[];
-
 void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag);
 void append_conflicts_hint(struct strbuf *msgbuf);
 int message_is_empty(const struct strbuf *sb,