lib/test_printf: Add empty module_exit function

Currently the test_printf module does not have an exit function, this
prevents the module from being unloaded.  If we cannot unload the
module we cannot run the tests a second time.

Add an empty exit function.
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>

lib/test_printf.c

@@ -615,5 +615,11 @@ test_printf_init(void)
 
 module_init(test_printf_init);
 
+static void __exit test_printf_exit(void)
+{
+}
+
+module_exit(test_printf_exit);
+
 MODULE_AUTHOR("Rasmus Villemoes <linux@rasmusvillemoes.dk>");
 MODULE_LICENSE("GPL");