fcntl34: Replace pthread_yield() with sched_yield()

sched_yield() is specified in POSIX, helps compiling with musl

home/root/work/ltp/testcases/kernel/syscalls/fcntl/fcntl34.c:88: undefined reference to `pthread_yield'
collect2: error: ld returned 1 exit status
make: *** [<builtin>: fcntl34] Error 1

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/testcases/kernel/syscalls/fcntl/fcntl34.c b/testcases/kernel/syscalls/fcntl/fcntl34.c
index c72951e..a1c0c44 100644
--- a/testcases/kernel/syscalls/fcntl/fcntl34.c
+++ b/testcases/kernel/syscalls/fcntl/fcntl34.c
@@ -23,6 +23,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <pthread.h>
+#include <sched.h>
 
 #include "lapi/fcntl.h"
 #include "tst_test.h"
@@ -84,7 +85,7 @@
 		if (fcntl(fd, F_OFD_SETLKW, &lck) == -1)
 			tst_brk(TBROK | TERRNO, "fcntl() failed");
 
-		pthread_yield();
+		sched_yield();
 	}
 
 	SAFE_CLOSE(fd);