lttng-tools: update hash

Fixes:
http://autobuild.buildroot.net/results/4c0/4c0f8583986d72fbc879d21bef65246bc9b4a732/
http://autobuild.buildroot.net/results/db4/db4011b3444c71942b1a269dd16b18652fd22d97/
http://autobuild.buildroot.net/results/220/220ef41676096bafd2e38f4fbb892eb0a7d654d8/
http://autobuild.buildroot.net/results/545/545371f5a39c85aba0d15a41cdc46e1cf6c1ba9f/



And many more.

Upstream silently updated the tarball on 30/10, breaking the build. The only
difference between the old and new tarballs is that the version number is
fixed:

diff -urpN old/lttng-tools-2.5.1/include/version.h new/lttng-tools-2.5.1/include/version.h
--- old/lttng-tools-2.5.1/include/version.h     2014-10-20 21:47:51.000000000 +0200
+++ new/lttng-tools-2.5.1/include/version.h     2014-10-31 16:33:17.000000000 +0100
@@ -20,7 +20,7 @@
 #ifndef VERSION_H
 #define VERSION_H

-#define GIT_VERSION "heads/stable-2.5-0-g56eb774"
+#define GIT_VERSION "2.5.1"

The copy of the old tarball on sources.buildroot.net has also been removed.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>