tracing/ftrace: make trace_find_cmdline() generally available

Impact: build fix

On !CONFIG_CONTEXT_SWITCH_TRACER trace_find_cmdline() is not defined:

 kernel/trace/trace_output.c: In function 'trace_ctxwake_print':
 kernel/trace/trace_output.c:499: error: implicit declaration of function 'trace_find_cmdline'
 kernel/trace/trace_output.c:499: warning: assignment makes pointer from integer without a cast

Move it to the generic section in trace.h.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/trace.h

@@ -469,10 +469,10 @@ struct tracer_switch_ops {
 	void				*private;
 	struct tracer_switch_ops	*next;
 };
-
-char *trace_find_cmdline(int pid);
 #endif /* CONFIG_CONTEXT_SWITCH_TRACER */
 
+extern char *trace_find_cmdline(int pid);
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 extern unsigned long ftrace_update_tot_cnt;
 #define DYN_FTRACE_TEST_NAME trace_selftest_dynamic_test_func