Cleanup: bt_clock_snapshot_set_raw_value is now a static inline

Cleanup: bt_clock_snapshot_set_raw_value is now a static inline

This fixes the following Clang warning:
  In file included from clock-class.c:32:
  ../../include/babeltrace/trace-ir/clock-snapshot-internal.h:98:1: error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
  BT_HIDDEN
  ^
  ../../include/babeltrace/babeltrace-internal.h:77:34: note: expanded from macro 'BT_HIDDEN'
  #define BT_HIDDEN __attribute__((visibility("hidden")))
                                   ^
  ../../include/babeltrace/trace-ir/clock-snapshot-internal.h:69:6: note: previous definition is here
  void bt_clock_snapshot_set_raw_value(struct bt_clock_snapshot *clock_snapshot,
       ^
  1 error generated.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>