libtraceevent: Fix lib installation with O=

When we use 'O=' with make to build libtraceevent in a separate folder
it fails to install libtraceevent.a and libtraceevent.so.1.1.0 with the
error:

  INSTALL  /home/sudip/linux/obj-trace/libtraceevent.a
  INSTALL  /home/sudip/linux/obj-trace/libtraceevent.so.1.1.0

  cp: cannot stat 'libtraceevent.a': No such file or directory
  Makefile:225: recipe for target 'install_lib' failed
  make: *** [install_lib] Error 1

I used the command:

  make O=../../../obj-trace DESTDIR=~/test prefix==/usr  install

It turns out libtraceevent Makefile, even though it builds in a separate
folder, searches for libtraceevent.a and libtraceevent.so.1.1.0 in its
source folder.

So, add the 'OUTPUT' prefix to the source path so that 'make' looks for
the files in the correct place.

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/20191115113610.21493-1-sudipm.mukherjee@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>