Implement ioprio_get and ioprio_set decoding

* ioprio.c: New file.
* Makefile.am (strace_SOURCES): Add ioprio.c.
* linux/dummy.h (sys_ioprio_get, sys_ioprio_set): Remove.
* linux/syscall.h (sys_ioprio_get, sys_ioprio_set): New prototypes.
diff --git a/linux/syscall.h b/linux/syscall.h
index 17b9431..316113e 100644
--- a/linux/syscall.h
+++ b/linux/syscall.h
@@ -121,6 +121,8 @@
 int sys_io_setup();
 int sys_io_submit();
 int sys_ioctl();
+int sys_ioprio_get();
+int sys_ioprio_set();
 int sys_ipc();
 int sys_kexec_load();
 int sys_kill();