Makefile: mark git-maintenance as a builtin

Makefile: mark git-maintenance as a builtin

We normally get the list of builtin commands by expanding BUILTIN_OBJS.
But for commands which are embedded inside another's source file (e.g.,
cmd_show() in builtin/log.c), the Makefile needs to be told explicitly
about them.

Since cmd_maintenance() is inside buitin/gc.c, it should be listed
explicitly in the BUILT_INS list in the Makefile. Not doing so isn't
_too_ tragic, as it simply means we will not make a git-maintenance
symlink in libexec/git-core. Since we encourage people to use the "git
foo" form, even in scripts which have put libexec into their PATH,
nobody seems to have noticed.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>