uatomic/generic: Fix redundant declaration warning

abort(3) was explicitly declared external to avoid including
<stdlib.h>. However, this emit a redundant declaration warning if it was
already declared before including <urcu/uatomic.h>.

Fix this by including <stdlib.h> and not declaring abort().

Change-Id: If9557814c311e2b531e85fec8c41788462338fe4
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>