Makefile: mark "check" target as .PHONY

Fix a bug in 44c9e8594e (Fix up header file dependencies and add sparse checking rules, 2005-07-03), we never marked the phony "check" target as such. Perhaps we should just remove it, since as of a combination of 912f9980d2 (Makefile: help people who run 'make check' by mistake, 2008-11-11) 0bcd9ae85d (sparse: Fix errors due to missing target-specific variables, 2011-04-21) we've been suggesting the user run "make sparse" directly. But under that mode it still does something, as well as directing the user to run "make test" under non-sparse. So let's punt that and narrowly fix the PHONY bug. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>