Have lexer not require <unistd.h> on MSVC

Required for `open`, `close`, `read`, and `STDIN_FILENO`,
which are defined elsewhere on MSVC.

src/asm/lexer.c

@@ -19,7 +19,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifndef _MSC_VER
 #include <unistd.h>
+#endif
 
 #include "extern/utf8decoder.h"
 #include "platform.h" /* For `ssize_t` */