lsusb example: make print_devs() static

lsusb.c:26: warning: no previous prototype for ‘print_devs’
diff --git a/examples/lsusb.c b/examples/lsusb.c
index 7e9f48e..317adf0 100644
--- a/examples/lsusb.c
+++ b/examples/lsusb.c
@@ -22,7 +22,7 @@
 
 #include <libusb/libusb.h>
 
-void print_devs(libusb_device **devs)
+static void print_devs(libusb_device **devs)
 {
 	libusb_device *dev;
 	int i = 0;