update .gitignore

ignore auto-generated opal/mca/pmix/pmix112/pmix/include/pmix/autogen/config.h.in
this is a one-off fix for the v2.x branch

Author: ggouaillardet

.gitignore

@@ -301,6 +301,7 @@ opal/mca/hwloc/hwloc*/hwloc/include/private/autogen/config.h
 opal/mca/installdirs/config/install_dirs.h
 
 opal/mca/pmix/pmix112/pmix/include/pmix/autogen/config.h
+opal/mca/pmix/pmix112/pmix/include/pmix/autogen/config.h.in
 opal/mca/pmix/pmix112/pmix/include/pmix/pmix_common.h
 opal/mca/pmix/pmix112/pmix/include/private/autogen/config.h
 opal/mca/pmix/pmix112/pmix/include/private/autogen/config.h.in