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/Makefile.am b/Makefile.am
index 26869fb..65929dc 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -25,6 +25,7 @@
 	inotify.c	\
 	io.c		\
 	ioctl.c		\
+	ioprio.c	\
 	ipc.c		\
 	kexec.c		\
 	loop.c		\