tracing/probes: Make reserved_field_names static

sparse complains:
  CHECK   kernel/trace/trace_probe.c
kernel/trace/trace_probe.c:16:12: warning: symbol 'reserved_field_names' was not declared. Should it be static?

Yes, it should be static.

Link: http://lkml.kernel.org/r/2478.1552380778@turing-policeSigned-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_probe.c

@@ -13,7 +13,7 @@
 
 #include "trace_probe.h"
 
-const char *reserved_field_names[] = {
+static const char *reserved_field_names[] = {
 	"common_type",
 	"common_flags",
 	"common_preempt_count",