gnu: openmpi: Add "debug" output.

This is necessary for one mechanism for attaching parallel debuggers
and is also relevant for the runtime backtrace facility.

* gnu/packages/mpi.scm (openmpi)[outputs]: New field.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/mpi.scm

@@ -131,6 +131,7 @@ (define-public openmpi
     (native-inputs
      `(("pkg-config" ,pkg-config)
        ("perl" ,perl)))
+    (outputs '("out" "debug"))
     (arguments
      `(#:configure-flags `("--enable-mpi-ext=affinity" ;cr doesn't work
                            "--enable-memchecker"