ftrace: Hide cached module code for !CONFIG_MODULES

ftrace: Hide cached module code for !CONFIG_MODULES

When modules are disabled, we get a harmless build warning:

kernel/trace/ftrace.c:4051:13: error: 'process_cached_mods' defined but not used [-Werror=unused-function]

This adds the same #ifdef around the new code that exists around
its caller.

Link: http://lkml.kernel.org/r/20170710084413.1820568-1-arnd@arndb.de
Fixes: d7fbf8df7ca0 ("ftrace: Implement cached modules tracing on module load")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>