tracing: Have stack tracer compile when MCOUNT_INSN_SIZE is not defined

tracing: Have stack tracer compile when MCOUNT_INSN_SIZE is not defined

commit b8299d362d0837ae39e87e9019ebe6b736e0f035 upstream.

On some archs with some configurations, MCOUNT_INSN_SIZE is not defined, and
this makes the stack tracer fail to compile. Just define it to zero in this
case.

Link: https://lore.kernel.org/r/202001020219.zvE3vsty%lkp@intel.com
Fixes: 4df297129f622 ("tracing: Remove most or all of stack tracer stack size from stack_max_size")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>