x86/eisa: Add missing include

The seperation of the EISA init missed to include linux/io.h which breaks
the build with some special configurations.

Reported-by: Ingo Molnar <mingo@kernel.org>
Fixes: f7eaf6e0 ("x86/boot: Move EISA setup to a separate file")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>