Build fix: missing initializer for member 'rotation_positions'

gcc 5.4.0 complains that:
  main.cpp: In function 'ssize_t relay_unpack_rotate_streams_header(const lttng_buffer_view*, lttcomm_relayd_rotate_streams*)':
  main.cpp:2547:2: warning: missing initializer for member 'lttcomm_relayd_rotate_streams::rotation_positions' [-Wmissing-field-initializers]

The structure's members are initialized one by one.

At the same time, the use of the address of a packed
member (stream_count) is eliminated, which fixes another unrelated
warning emited by clang.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5fd90d75cc6e0ba17350fc8092929f476e93757e