scalar Makefile: use "The default target of..." pattern
Make the "contrib/scalar/Makefile" be stylistically consistent with
the top-level "Makefile" in first declaring "all" to be the default
rule, followed by including other Makefile snippets.
This adjusts code added in
0a43fb22026 (scalar: create a rudimentary
executable, 2021-12-03), it further ensures that when we add another
"include" file in a subsequent commit that the included file won't be
the one to define our default target.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>