tracing: Remove unneeded pointer casts

Cleaup uneeded casts from void * to char * in syscalls tracing file.

Reported-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>