sessiond: make thread_rotation function static

sessiond: make thread_rotation function static

Fixes:

      CC       rotation-thread.o
    /home/smarchi/src/lttng-tools/src/bin/lttng-sessiond/rotation-thread.c:799:7: error: no previous declaration for ‘thread_rotation’ [-Werror=missing-declarations]
     void *thread_rotation(void *data)
           ^~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ia95212a53c658bce5dc6df155eb43ee29b7980c6
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>