cmListFileLexer: Add missing include to avoid possible pointer truncation

The `cmsys/Enconding.h` include had a typo in its surrounding 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.
8 jobs for fix-pointer-truncation
Status Job ID Name Coverage
  External
passed ghostflow-check-master

passed ghostflow-stager

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passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager