btl/usnic: fix compiler warning

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

opal/mca/btl/usnic/btl_usnic_component.c

@@ -90,7 +90,7 @@
 opal_recursive_mutex_t btl_usnic_lock =  OPAL_RECURSIVE_MUTEX_STATIC_INIT;
 
 /* RNG buffer definition */
-opal_rng_buff_t opal_btl_usnic_rand_buff = {0};
+opal_rng_buff_t opal_btl_usnic_rand_buff = {{0}};
 
 /* simulated clock */
 uint64_t opal_btl_usnic_ticks = 0;