stacktrace: fix modular build, export print_stack_trace and save_stack_trace

fix:

ERROR: "print_stack_trace" [kernel/backtracetest.ko] undefined!
ERROR: "save_stack_trace" [kernel/backtracetest.ko] undefined!

Signed-off-by: Ingo Molnar <mingo@elte.hu>

and fix:

  Building modules, stage 2.
  MODPOST 376 modules
ERROR: "print_stack_trace" [kernel/backtracetest.ko] undefined!
make[1]: *** [__modpost] Error 1

Signed-off-by: Ingo Molnar <mingo@elte.hu>