trace-cmd: fix site

Replace $(BR2_KERNEL_MIRROR) by https://git.kernel.org/pub, which
fixes the download of this package:

>>> trace-cmd trace-cmd-v2.6.1 Downloading
Initialized empty Git repository in /home/thomas/dl/trace-cmd/git/.git/
Fetching all references
fatal: repository 'https://cdn.kernel.org/pub/scm/linux/kernel/git/rostedt/trace-cmd.git/' not found
Detected a corrupted git cache.
Removing it and starting afresh.
Initialized empty Git repository in /home/thomas/dl/trace-cmd/git/.git/
Fetching all references
fatal: repository 'https://cdn.kernel.org/pub/scm/linux/kernel/git/rostedt/trace-cmd.git/' not found
Detected a corrupted git cache.
This is the second time in a row; bailing out
--2018-11-11 21:08:00--  http://sources.buildroot.net/trace-cmd/trace-cmd-trace-cmd-v2.6.1.tar.gz


Resolving sources.buildroot.net (sources.buildroot.net)... 104.25.210.19, 104.25.211.19, 2606:4700:20::6819:d313, ...
Connecting to sources.buildroot.net (sources.buildroot.net)|104.25.210.19|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1859835 (1.8M) [application/x-gtar-compressed]

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>