lowercase include header name in syshead.h

lowercase include header name in syshead.h

Cross compiling for Windows is broken since commit
739fa9881f12e67dc8b9cadc7230e59e7fe42423 added the mixed
case header name "NtDDNdis.h" to the file. While this header
exists in a MinGW build environment it's lowercase there.

Windows doesn't mind the case of a file name, but Linux does.
So, lowercasing the filename will make openvpn build in both
worlds.

Signed-off-by: Heiko Hund <heiko.hund@sophos.com>
Acked-by: David Sommerseth <davids@redhat.com>
Signed-off-by: David Sommerseth <davids@redhat.com>