Fix variable declaration of the clock source v2

Fix variable declaration of the clock source v2

The ust_clock_source was static in clock.h header file.
Thus, tracectl.c was not able to modify that variable for
the trace clock read function. Bad timestamp was always
returned if CLOCK_TRACE was not available.

v2:
	Comment by Mathieu Desnoyers, set clock source variable
	to extern in clock.h

Signed-off-by: David Goulet <david.goulet@polymtl.ca>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Nils Carlson <nils.carlson@ericsson.com>