libtraceevent: fix qa issue reference to tmpdir

Make sure that the OE provided CFLAGS are passed to the compiler so
that the -fdebug-prefix-map and -fmacro-prefix-map options are
actually active.

Fixes:
| WARNING: libtraceevent-1.7.2-r0 do_package_qa: QA Issue: File /usr/lib/libtraceevent.a in package libtraceevent-staticdev contains reference to TMPDIR [buildpaths]
| WARNING: libtraceevent-1.7.2-r0 do_package_qa: QA Issue: File /usr/lib/.debug/libtraceevent.so.1.7.2 in package libtraceevent-dbg contains reference to TMPDIR [buildpaths]

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>