examples: standalone: Fix build with LLVM toolchain

When building the standalone example with llvm, the link step fails:

examples/standalone/libstubs.o: In function `dummy':
include/_exports.h:10: undefined reference to `jt'
include/_exports.h:11: undefined reference to `jt'
include/_exports.h:12: undefined reference to `jt'
include/_exports.h:13: undefined reference to `jt'
include/_exports.h:14: undefined reference to `jt'
examples/standalone/libstubs.o:include/_exports.h:15:
  more undefined references to `jt' follow

Indeed, the standalone libstubs.o does use the jt symbol, but it was
marked 'static' in stubs.c. It's strange how gcc builds are working.

Signed-off-by: Alistair Delva <adelva@google.com>
Cc: Rick Chen <rick@andestech.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>