linux.mk: enable options for ktap

Certain tracing related options are required to be able to build ktapvm.ko, enable those.

Enable CONFIG_FUNTCTION_TRACER as otherwise, CONFIG_EVENT_TRACING won't stick. (Some
tracer needs to be enabled for this).

[Peter: add a note to ktap Config.in explaining this is done]
Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>