lock: include unistd.h for syscall function declaration

sys/syscall.h only defines syscall number constants but
not syscall function.
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/lock.h

@@ -13,6 +13,7 @@
 #ifdef CR_NOGLIBC
 # include "syscall.h"
 #else
+# include <unistd.h>
 # include <sys/syscall.h>
 static inline long sys_futex(void *addr1, int op, int val1,
                             struct timespec *timeout, void *addr2, int val3)