trace-cmd: fix largefile build problem

The source files of trace-cmd already define _LARGEFILE64_SOURCE, but
Buildroot passes it again on the build command line through CFLAGS,
which causes a build error. We fix this by filtering out our
definition of _LARGEFILE64_SOURCE from the CFLAGS before passing them
to the trace-cmd Makefile.

Fixes the warning visible at:

 http://autobuild.buildroot.net/results/1c4567a230940a5287d03e2a5c2c7afbdc64cd36/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>