kernel/mingw32: use pre-set headers dir

There is a variable that contains the headers directory,
so there's no need to re-compute it again.

[Yann E. MORIN: split the original patch]

reporter_name="Bart vdr. Meulen <bartvdrmeulen@gmail.com>"

reporter_url=""

reporter_comment="Config to build cross-compiler with mingw as kernel type"