selftests: ftrace: Fix trigger-mod to run without syscall trace

Since histogram trigger id.syscall depends on CONFIG_FTRACE_SYSCALLS,
a testcase in trigger-modifier test fails if that config is disabled.

Fix this bug by using flexible pattern to check the histogram output.

Link: http://lkml.kernel.org/r/147928402670.22982.15589445159052676877.stgit@devbox



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>