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>

# Default test suite configuration

# Default DejaGnu configuration

DG_TOOLNAME = gcc

DG_TARGET_HOSTNAME = 127.0.0.1

DG_TARGET_USERNAME = root

# Default tests

DG_C_TESTS = 

DG_CPP_TESTS =