maint: use consistent file name for threading notes

* daemon/THREADING.txt: Rename...
* daemon/THREADS.txt: ...to match qemu thread notes.
* daemon/Makefile.am (EXTRA_DIST): Reflect rename.

daemon/Makefile.am

@@ -44,7 +44,7 @@ EXTRA_DIST =						\
 	libvirtd.lxc.logrotate.in                       \
 	libvirtd.uml.logrotate.in                       \
 	test_libvirtd.aug                               \
-	THREADING.txt					\
+	THREADS.txt					\
 	libvirtd.pod.in					\
 	libvirtd.8.in					\
 	libvirtd.stp					\