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.

Source/LexerParser/cmListFileLexer.c

@@ -766,7 +766,7 @@ Modify cmListFileLexer.c:
 
 /* IWYU pragma: no_forward_declare yyguts_t */
 
-#ifdef WIN32
+#ifdef _WIN32
 #include "cmsys/Encoding.h"
 #endif
 

Source/LexerParser/cmListFileLexer.in.l

@@ -18,7 +18,7 @@ Modify cmListFileLexer.c:
 
 /* IWYU pragma: no_forward_declare yyguts_t */
 
-#ifdef WIN32
+#ifdef _WIN32
 #include "cmsys/Encoding.h"
 #endif