romio321: ensure to distribute ompi_grequestx.h

Refs https://github.com/open-mpi/ompi/issues/6227.  Thanks to
George Marselis for reporting.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
(cherry picked from commit 62321be186dd7d3efcedc2e801f226f6660ea0c4)

ompi/mca/io/romio321/romio/Makefile.am

@@ -68,6 +68,9 @@ AM_CPPFLAGS += -I$(top_builddir)/include -I$(top_srcdir)/include
 noinst_HEADERS += include/mpio.h
 noinst_HEADERS += include/io_romio_conv.h
 
+# Included for Open MPI's --enable-grequest-extensions feature.
+noinst_HEADERS += include/ompi_grequestx.h
+
 # ------------------------------------------------------------------------
 
 SUBDIRS =