trace: ftrace_dump_on_oops[] is not exported, make it static

trace: ftrace_dump_on_oops[] is not exported, make it static

The ftrace_dump_on_oops string is not used outside of trace.c so
make it static to avoid the export warning from sparse:

kernel/trace/trace.c:141:6: warning: symbol 'ftrace_dump_on_oops' was not declared. Should it be static?

Fixes: dd293df6395a2 ("tracing: Move trace sysctls into trace.c")
Link: https://patch.msgid.link/20260106231054.84270-1-ben.dooks@codethink.co.uk
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>