Fix the following compiler error:

v4l2-i2c-drv.h:72: error: implicit declaration of function 'v4l2_i2c_attach'

Also, prevent multiple inclusions of v4l2-i2c-drv.h
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>