Bugfix: avoid link error in Feather-Trace on x86

If no events are defined but Feater-Trace support is enabled, then the current
implementation generates a link error because the __event_table sections is
absent.

> arch/x86/built-in.o: In function `ft_disable_all_events':
> (.text+0x242af): undefined reference to `__start___event_table'

As a simple work around, we force zero-element array to always be "allocated"
in the __event_table section. This ensures that we end up with a zero-byte
section if no events are enabled, and does not affect the layout of the section
if events are present.

> bbb@ludwig:~/dev/litmus2010$ nm vmlinux | grep event_table
> ffffffff81950cdc D __event_table_dummy
> ffffffff81950cdc A __start___event_table
> ffffffff81950cdc A __stop___event_table

0 files changed, 0 insertions, 0 deletions