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>

 #!/bin/sh

 # Needed for systems without gettext

 $* -xc -o /dev/null - > /dev/null 2>&1 << EOF

 #include <libintl.h>

 int main()

 {

 	gettext("");

 	return 0;

 }

 EOF

 if [ ! "$?" -eq "0"  ]; then

 	echo -DKBUILD_NO_NLS;

 fi