libc/mingw: fix mingw source dirs

Someof the mingw32 source tarballs have an appended '-src' after the
version.

Since changeset #6e1412ba8da9 (scripts/functions: force extract folder
to archive basename), it means mingw tarballs get extracted in a directory
ending with '-src'.

Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="YEM"

 reporter_url="http://ymorin.is-a-geek.org/"

 reporter_comment=""