coll/ml: declare a global variable as static to avoid an uninitialized common symbol.

ompi/mca/coll/ml/coll_ml_allocation.c

@@ -17,7 +17,7 @@
 #include "coll_ml_inlines.h"
 #include "coll_ml_allocation.h"
 
-long memory_buffer_index;
+static long memory_buffer_index;
 
 mca_bcol_base_memory_block_desc_t *mca_coll_ml_allocate_block(struct mca_coll_ml_component_t *ml_component,
                                                    mca_bcol_base_memory_block_desc_t *ml_memblock)