bcm2708: alert mach/gpio.h:gpio_to_irq() users to bogusness

people who upgrade from earlier version may still use the *ex*
gpio_to_irq() of *ex* <mach/gpio.h>. in order to alert them to the
bogusness of the situation, re-add <mach/gpio.> - for the sole purpose
of placing a #error in it which clarifies what's to be done.