m68k: hardirq_count() only need preempt_mask.h

The m68k irqflags implementation needs to check hardirq
context in some cases.

As it is a very low level header file, it's better to
include preempt_mask.h rather than hardirq.h when the
only purpose is to use irq context APIs. This way we
can avoid future header circular dependencies when
vtime.h will expand to use static keys.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>