Fix: perform lazy initialization of getenv common lib

The ust-dl wrapper ends up calling into the getenv common lib very early
from library constructors through use of dlopen by the tracepoint
instrumentation code.

This happens before the getenv common lib is explicitly initialized by
liblttng-ust's contructor.

Perform lazy initialization for those early use by constructor
scenarios.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I9f84bb776a62e03b11db0386e267df00eb165ddd