Build with -fshort-wchar
As described in https://bugzilla.redhat.com/show_bug.cgi?id=2310759, commit 189200d0b0f6fff473d302880d9569f45d4d8c4d in gnu-efi 3.18 breaks sbvarsign unless sbsigntools is compiled with -fshort-wchar, so let's build it with -fshort-wchar to make sure sbvarsign works again.