libtraceevent: Copy pkg-config file to output folder when using O=

When we use 'O=' with make to build libtraceevent in a separate folder
it still copies 'libtraceevent.pc' to its source folder. Modify the
Makefile so that it uses the output folder to copy the pkg-config file
and install from there.

Signed-off-by: Sudipm Mukherjee <sudipm.mukherjee@gmail.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20191115113610.21493-2-sudipm.mukherjee@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>