syntax-check: Enforce sc_prohibit_backslash_alignment everywhere

syntax-check: Enforce sc_prohibit_backslash_alignment everywhere

Basically all files in the repository are already passing the
check, except for syntax-check.mk itself. Fix that, and stop
limiting the files on which the test is performed.

These changes have been generated by running

  $ sed -Ei 's/[ '$'\t'']+\\$/ \\/g' $(git grep -El '[ '$'\t'']+\\$')

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>