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>

 # m68k specific configuration file

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORT_CPU

 ##

 ## help The m68k architecture