ftrace: split out hardirq ftrace code into own header

Impact: moving of function prototypes into own header file

ftrace.h is too big of a file for hardirq.h, and some archs will fail
to build because of the include dependencies not being met.

This patch pulls out the required prototypes for hardirq.h into a smaller
and safer ftrace_irq.h file.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>