tracing: Use outer () on __get_str() definition

tracing: Use outer () on __get_str() definition

__get_str(str)'s definition includes a (char *) operator
overloading that is not protected with outer ().

This patch adds () around __get_str()'s definition, enabling
some code cleanup.

Link: http://lkml.kernel.org/r/20ac1a10c2ec4ccd23e4a8ef34101fb6e4157d37.1467407618.git.bristot@redhat.com
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: Ingo Molnar <mingo@redhat.com>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>