makefile/meson: add 'check-headers' as alias for 'hdr-check'

The 'hdr-check' target in Meson and makefile is used to check if headers
can be compiled individually. The naming however isn't readable as 'hdr'
is not a common shortforme for 'header', neither is it an abbreviation.

Let's introduce 'check-headers' as an alternative target for 'hdr-check'
and add a `TODO` to deprecate the latter after 2 releases. Since this
is an internal tool, we can use a shorter deprecation cycle.

Change existing usage of 'hdr-check' in 'ci/run-static-analysis.sh' to
also use 'check-headers'.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -3326,8 +3326,10 @@ HCC = $(HCO:hco=hcc)
 $(HCO): %.hco: %.hcc $(GENERATED_H) FORCE
 	$(QUIET_HDR)$(CC) $(ALL_CFLAGS) -o /dev/null -c -xc $<
 
-.PHONY: hdr-check $(HCO)
+# TODO: deprecate 'hdr-check' in lieu of 'check-headers' in Git 2.51+
+.PHONY: hdr-check check-headers $(HCO)
 hdr-check: $(HCO)
+check-headers: hdr-check
 
 .PHONY: style
 style: