Only prototype function if we need it

This commit was SVN r5014.

src/mca/pls/fork/pls_fork_module.c

@@ -50,7 +50,9 @@
 
 extern char **environ;
 
+#if OMPI_HAVE_POSIX_THREADS && OMPI_THREADS_HAVE_DIFFERENT_PIDS && OMPI_ENABLE_PROGRESS_THREADS
 static int orte_pls_fork_launch_threaded(orte_jobid_t);
+#endif
 
 
 orte_pls_base_module_1_0_0_t orte_pls_fork_module = {