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

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>