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>
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index ffc16ab..89a56d7 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -4,7 +4,7 @@
 #include <linux/preempt.h>
 #include <linux/smp_lock.h>
 #include <linux/lockdep.h>
-#include <linux/ftrace.h>
+#include <linux/ftrace_irq.h>
 #include <asm/hardirq.h>
 #include <asm/system.h>