package/trace-cmd: update download url

The git repository has been moved to http://www.kernel.org/pub/ which
allows to use $(BR2_KERNEL_MIRROR)

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/trace-cmd/trace-cmd.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 TRACE_CMD_VERSION = trace-cmd-v2.2.1
-TRACE_CMD_SITE = http://git.kernel.org/cgit/linux/kernel/git/rostedt/trace-cmd.git
+TRACE_CMD_SITE = $(BR2_KERNEL_MIRROR)/scm/linux/kernel/git/rostedt/trace-cmd.git
 TRACE_CMD_SITE_METHOD = git
 TRACE_CMD_INSTALL_STAGING = YES
 TRACE_CMD_LICENSE = GPLv2 LGPLv2.1