fwk_interrupt: add missing declaration of exported functions

Add missing declaration of exported functions fwk_interrupt_init()
and fwk_interrupt_set_isr_fault().

fwk_interrupt_init() is an effective exported API hence declared from
fwk_interrupt.h.

fwk_interrupt_set_isr_fault() is a test only API hence declared from
internal/fwk_interrupt.h.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>