tracing: Put trace_puts() comment above trace_puts() macro for kernel doc

Kernel-doc gives the following warning:

  DOCPROC Documentation/DocBook/kernel-api.xml
Warning(/include/linux/kernel.h:590): No description found for parameter 'ip'
Warning(/include/linux/kernel.h:590): No description found for parameter 'ip'

Due to the externs between the the comment and the trace_puts() macro.
This is fixed by moving the externs below the macro and keeping the
macro and comment directly together.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>