export: declare ksymtab symbols

sparse complains about any __ksymtab symbols with the following:

 warning: symbol '__ksymtab_...' was not declared. Should it be static?

due to Andi's patch making it non-static.

Mollify sparse by declaring the symbol extern, otherwise we get
drowned in sparse warnings for anything that uses EXPORT_SYMBOL
in the sources, making it easy to miss real warnings.

Fixes: e0f244c6

 ("asmlinkage, module: Make ksymtab [...] __visible")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>