d/ohmd: Fix warning by including right header

src/xrt/drivers/ohmd/oh_device.c

@@ -10,6 +10,7 @@
 
 #include <math.h>
 #include <stdio.h>
+#include <unistd.h>
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>