Add GPL-3 code subprojects/cairo/util/ in copyright.whitelist

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

debian/apertis/copyright.whitelist

 # Test program, not affecting artifacts
 debian/tests/run-with-display
 tests/testscrolledge.c
+# GPL-3 code, not used to build the Debian packages
+subprojects/cairo/util/cairo-fdr/fdr.c
+subprojects/cairo/util/cairo-trace/*
+subprojects/cairo/util/cairo-trace/lookup-symbol.c
+subprojects/cairo/util/cairo-trace/lookup-symbol.h
+subprojects/cairo/util/cairo-trace/trace.c