Fix compiler warning in mca_mpool_basic_create()

[jsquyres]

Per #7211 (comment), slightly shift the implementation of mca_mpool_basic_create() to be more correct.

[jsquyres]

@bosilca Were you thinking something like this? If so, why are we swizzling through a (uintptr_t) instead of just storing a (void*) directly?

diff --git a/opal/mca/mpool/base/mpool_base_basic.c b/opal/mca/mpool/base/mpool_base_
index fba7e6fed7..e11f5273ea 100644
--- a/opal/mca/mpool/base/mpool_base_basic.c
+++ b/opal/mca/mpool/base/mpool_base_basic.c
@@ -81,7 +81,15 @@ static void mca_mpool_base_basic_finalize (struct mca_mpool_base_m
     free (mpool);
 }
 
+static void *mca_mpool_base_basic_base(mca_mpool_base_module_t *mpool)
+{
+    mca_mpool_base_basic_module_t *basic_module = (mca_mpool_base_basic_module_t *) 
+
+    return (void*) basic_module->ptr;
+}
+
 static mca_mpool_base_module_t mca_mpool_basic_template = {
+    .mpool_base = mca_mpool_base_basic_base,
     .mpool_alloc = mca_mpool_base_basic_alloc,
     .mpool_free = mca_mpool_base_basic_free,
     .mpool_finalize = mca_mpool_base_basic_finalize,
@@ -100,7 +108,6 @@ mca_mpool_base_module_t *mca_mpool_basic_create (void *base, size
 
     OBJ_CONSTRUCT(&basic_module->lock, opal_mutex_t);
 
-    basic_module->super.mpool_base = base;
     basic_module->ptr = (uintptr_t) base;
     basic_module->size = basic_module->avail = size;
     basic_module->min_align = min_align;