Fix: libc-wrapper: undef temporary token rather than value

Fix: libc-wrapper: undef temporary token rather than value

The lttng-ust malloc wrapper defines pthread_mutex_lock/unlock
preprocessor tokens to ust_malloc_spin_lock/unlock around the definition
of a TLS variable, which uses pthread mutexes when relying on the
pthread key fallback. Undefining those tokens should be done on the
preprocessor token, rather than its value.

Signed-off-by: Christophe Bedard <bedard.christophe@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I2053b79c88000e272c29b25ca105b1352ecfabd7