ktap: add configs for elfutils and ffi

libelf (elfutils) is required for resolving symbols in DSO and for sdt (taken
from ktap docs). The earlier v0.4 recipe built ktap against libelf if it was
found, thus enable it per default.

FFI needs to be enabled for both userspace and kernel module. Currently, it is
only supported for x86_64 (enforced when building ktap-module).

Signed-off-by: Anders Darander <anders@chargestorm.se>