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: Ingo Molnar <mingo@elte.hu>