Fix: java agent 32-bit pointer to jlong warning

Fix: java agent 32-bit pointer to jlong warning

Fix the following compiler warning on 32-bit:

15:07:12 lttng_ust_context.c: In function 'Java_org_lttng_ust_agent_context_LttngContextApi_registerProvider':
15:07:12 lttng_ust_context.c:377:17: warning: cast from pointer to
integer of different size [-Wpointer-to-int-cast]
15:07:12   provider_ref = (jlong) provider;

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