mesa: protect #include of unistd.h with _MSV_VER check

unistd.h is unix only.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>

src/mesa/program/program_lexer.l

@@ -21,7 +21,11 @@
  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  * DEALINGS IN THE SOFTWARE.
  */
+
+#ifndef _MSC_VER
 #include <unistd.h>
+#endif
+
 #include "main/glheader.h"
 #include "main/imports.h"
 #include "program/prog_instruction.h"