tracing: synth events: increase max fields count

Sometimes it is useful to construct larger synthetic trace events. Increase
'SYNTH_FIELDS_MAX' (maximum number of fields in a synthetic event) from 32 to
64.

Link: https://lkml.kernel.org/r/20210901135513.3087062-1-dedekind1@gmail.com

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Acked-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>