Fix "git help -a" terminal autosizing

Fix "git help -a" terminal autosizing

When I split out the builtin commands into their own files, I left the
include of <sys/ioctl.h> in git.c rather than moving it to the file that
needed it (builtin-help.c).

Nobody seems to have noticed, because everything still worked, but because
the TIOCGWINSZ macro was now no longer defined when compiling the
"term_columns()" function, it would no longer automatically notice the
terminal size unless your system used the ancient "COLUMNS" environment
variable approach.

Trivially fixed by just moving the header include to the file that
actually needs it.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>