coll/base: fix compiler warnings

Add some "const"s that needed to be applied here on the v4.1.x branch,
effectively by cherry-picking part of b65ec273074 from master.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

ompi/mca/coll/base/coll_base_util.c

@@ -257,7 +257,7 @@ static int free_vecs_callback(struct ompi_request_t **rptr) {
 }
 
 int ompi_coll_base_retain_datatypes_w( ompi_request_t *req,
-                                       ompi_datatype_t *stypes[], ompi_datatype_t *rtypes[]) {
+                                       ompi_datatype_t *const stypes[], ompi_datatype_t *const rtypes[]) {
     ompi_coll_base_nbc_request_t *request = (ompi_coll_base_nbc_request_t *)req;
     bool retain = false;
     ompi_communicator_t *comm = request->super.req_mpi_object.comm;

ompi/mca/coll/base/coll_base_util.h

@@ -174,8 +174,8 @@ int ompi_coll_base_retain_datatypes( ompi_request_t *request,
  * (will be cast internally).
  */
 int ompi_coll_base_retain_datatypes_w( ompi_request_t *request,
-                                       ompi_datatype_t *stypes[],
-                                       ompi_datatype_t *rtypes[]);
+                                       ompi_datatype_t * const stypes[],
+                                       ompi_datatype_t * const rtypes[]);
 
 /* File reading function */
 int ompi_coll_base_file_getnext_long(FILE *fptr, int *fileline, long* val);