cmListFileLexer: Add missing include to avoid possible pointer truncation
The cmsys/Enconding.h include in cmListFileLexer.c had a typo in its sorruonding ifdef, possibly causing a missing function declaration (cmsysEncoding_DupToWide). As this is C code, this resulted in the code compiling, but with a truncated return value, possibly causing crashes.
Found when building with clang.exe on Windows.