Make sure that the appropriate feature flags are set when
Rts.h is included, so that clockid_t is defined.

diff --git a/cbits/is-valid-utf8.c b/cbits/is-valid-utf8.c
index 01b3b41..c69596a 100644
--- a/libraries/bytestring/cbits/is-valid-utf8.c
+++ b/libraries/bytestring/cbits/is-valid-utf8.c
@@ -29,6 +29,7 @@ SUCH DAMAGE.
 */
 #pragma GCC push_options
 #pragma GCC optimize("-O2")
+#include "rts/PosixSource.h"
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>