barectf.c.j2: only declare `ts` variable when needed

barectf.c.j2: only declare `ts` variable when needed

This patch changes the `barectf.c.j2` template so that, for the
packet opening, packet closing, and event record header serialization
functions, the `ts` variable is only declared when it's used.

Without this patch, the C compiler can report such a warning:

    warning: unused variable ‘ts’ [-Wunused-variable]

In `barectf.c-macros.j2`, the open_close_func_preamble() macro is
changed to accept the timestamp packet feature in order to check either
the packet beginning or end timestamp field type feature.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>