dpfp_threaded: return instead of pthread_exit() to avoid Cygwin warning

examples/dpfp_threaded.c

diff --git a/examples/dpfp_threaded.c b/examples/dpfp_threaded.c
index e56ea88..c3cac73 100644
--- a/examples/dpfp_threaded.c
+++ b/examples/dpfp_threaded.c
@@ -91,7 +91,7 @@
 	}
 
 	printf("poll thread shutting down\n");
-	pthread_exit(NULL);
+	return NULL;
 }
 
 static int find_dpfp_device(void)