Fix: c99: static assert: clang build fails due to multiple typedef

Unlike c11, c99 does not allow redefinition of the same typedef, and
clang is strict about it. Building code with tracepoints with -std=c99
with clang fails with:

warning: redefinition of typedef 'lttng_ust_static_assert_Tracepoint_name_length_is_too_long' is a C11 feature [-Wtypedef-redefinition]

Fix this by placing the (potentially negative size) array as argument to
a function prototype instead.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I06b6edbcd93f43f349451c23b0520df59f4fb346