add unistd.h compat header for visual studio

Using:

https://stackoverflow.com/a/826027/262458

Revert 5019a201.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

dependencies

@@ -1 +1 @@
-Subproject commit 33a0af6701313fed2219029a069c712740233a77
+Subproject commit d1e454edf05a6e99e3730f9d74772e617d8333eb

src/sdl/expr-lex.cpp

@@ -470,12 +470,8 @@ 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*