trace: rebuild generated-events.o when configuration changes

Make sure to rebuild generated-events.o when ./configure options change.
This prevents linker errors when a stale generated-events.o gets linked
with code compiled against fresh headers.  For example, try building
with ./configure --enable-trace-backend=stderr followed by ./configure
--enable-trace-backend=dtrace.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
