Add sys/membarrier.h to the headers list

Signed-off-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Mihnea Firoiu <mihneafiroiu0@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #76

Author: StefanJum

Makefile.uk.musl.sched

@@ -4,6 +4,7 @@ LIBMUSL_SCHED_HDRS-y += $(LIBMUSL)/include/sys/membarrier.h
 LIBMUSL_SCHED_HDRS-y += $(LIBMUSL)/src/internal/pthread_impl.h
 LIBMUSL_SCHED_HDRS-y += $(LIBMUSL)/include/sched.h
 LIBMUSL_SCHED_HDRS-y += $(LIBMUSL)/include/string.h
+LIBMUSL_SCHED_HDRS-y += $(LIBMUSL)/include/sys/membarrier.h
 LIBMUSL_SCHED_HDRS-y += $(LIBMUSL)/src/internal/syscall.h
 
 LIBMUSL_SCHED_SRCS-y += $(LIBMUSL)/src/sched/affinity.c