windows: Don't try to include <unistd.h>.

This should enable compatibility with MSVC.

libserialport_internal.h

@@ -34,7 +34,6 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
-#include <unistd.h>
 #include <stdlib.h>
 #include <errno.h>
 #include <stdio.h>
@@ -63,6 +62,7 @@
 #include <sys/time.h>
 #include <time.h>
 #include <poll.h>
+#include <unistd.h>
 #endif
 #ifdef __APPLE__
 #include <CoreFoundation/CoreFoundation.h>