pmix: fix Makefile.am to correctly exclude autogenerated file from tarball

(back-ported from openpmix/openpmix@73daf58)

(cherry picked from commit open-mpi/ompi@f5a53b5)

Author: ggouaillardet

opal/mca/pmix/pmix112/pmix/include/Makefile.am

@@ -38,7 +38,9 @@ include_pmix_autogen_HEADERS = \
         include/pmix/autogen/pmix_config_top.h \
         include/pmix/autogen/pmix_config_bottom.h
 nodist_include_pmix_autogen_HEADERS = \
-        include/pmix/autogen/config.h \
+        include/pmix/autogen/config.h
+include_private_autogendir = $(includedir)/private/autogen
+nodist_include_private_autogen_HEADERS = \
         include/private/autogen/config.h
 
 endif ! PMIX_EMBEDDED_MODE