libc/mingw: 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="Alexander BIGGA"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-07/msg00000.html"

 reporter_comment=""