[SDL Front] MSVC doesn't have an unistd.h system file, closest is io.h, this will fix building the SDL frontend with msvc again.

src/sdl/expr-lex.cpp

@@ -470,8 +470,12 @@ int exprCol;
  * down here because we want the user's section 1 to have been scanned first.
  * The user has a chance to override it with an option.
  */
+#ifndef __MSC_VER
+#include <io.h>
+#else
 #include <unistd.h>
 #endif
+#endif
 
 #ifndef YY_EXTRA_TYPE
 #define YY_EXTRA_TYPE void*