samples: ftrace: Make some global variables static

samples: ftrace: Make some global variables static

smatch reports this representative issue
samples/ftrace/ftrace-ops.c:15:14: warning: symbol 'nr_function_calls' was not declared. Should it be static?

The nr_functions_calls and several other global variables are only
used in ftrace-ops.c, so they should be static.
Remove the instances of initializing static int to 0.

Link: https://lore.kernel.org/linux-trace-kernel/20230130193708.1378108-1-trix@redhat.com
Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>