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>

tools/testing/selftests/ftrace/test.d/trigger/trigger-hist-mod.tc

@@ -56,7 +56,7 @@ echo "Test histogram with syscall modifier"
 
 echo 'hist:keys=id.syscall' > events/raw_syscalls/sys_exit/trigger
 for i in `seq 1 10` ; do ( echo "forked" > /dev/null); done
-grep "id: sys_" events/raw_syscalls/sys_exit/hist > /dev/null || \
+grep "id: \(unknown_\|sys_\)" events/raw_syscalls/sys_exit/hist > /dev/null || \
     fail "syscall modifier on raw_syscalls/sys_exit did not work"