urcu.c: declare noop urcu_init() function non-static

The RCU library is sometimes used in constructor context. In these cases,
because it is impossible to predict whether the liburcu constructor has already
been called, an explicit call to the urcu_init function must be made.
Declaring the noop version of the urcu_init() function as static breaks code
that calls urcu_init() explicitly when CONFIG_AVOID_SIGNALS is true. To prevent
this, this patch makes the declaration non-static.

From: Pierre-Marc Fournier <pierre-marc.fournier@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>