gpio_emul: Fail at build time if ngpios is too big

Add BUILD_ASSERT to check max val of ngpios. If someone sets ngpios to
more than 32, and acts on pin 32 or higher, memory corruption will
result. This is only a problem with the emulator, real hardware
will have limits that are probably much smaller.

Signed-off-by: Jeremy Bettis <jbettis@google.com>