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>

Makefile

@@ -2925,6 +2925,7 @@ hdr-check: $(HCO)
 style:
 	git clang-format --style file --diff --extensions c,h
 
+.PHONY: check
 check: config-list.h command-list.h
 	@if sparse; \
 	then \