Fix compiling error when enable debug

[minsii]

What

ucc_tl_rccl_context_t *ctx was not defined but is used by ucc_assert. It caused compilation error (see below).

../../../../../src/components/tl/rccl/tl_rccl_coll.c: In function ‘ucc_tl_rccl_collective_sync’:
../../../../../src/components/tl/rccl/tl_rccl_coll.c:149:16: error: ‘ctx’ undeclared (first use in this function)
     ucc_assert(ctx->cfg.sync_type == UCC_TL_RCCL_COMPLETION_SYNC_TYPE_EVENT);

This commit fixes it. ctx will be defined when debug build is on (configure with --enable-debug)