pmix: update git ignore

allow AUTHORS and contrib/perf_tools/Makefile from the embedded PMIx

Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>

.gitignore

@@ -367,6 +367,8 @@ opal/mca/hwloc/base/static-components.h.new.struct
 
 opal/mca/installdirs/config/install_dirs.h
 
+!opal/mca/pmix/pmix*/pmix/AUTHORS
+!opal/mca/pmix/pmix*/pmix/contrib/perf_tools/Makefile
 opal/mca/pmix/pmix*/pmix/include/pmix/autogen/config.h
 opal/mca/pmix/pmix*/pmix/include/pmix/autogen/config.h.in
 opal/mca/pmix/pmix*/pmix/src/include/private/autogen/config.h.in