GenericI2CSensor: do not fail compilation without PIOS_I2C_INCLUDE

This module makes the code not compile without the I2C include flag.
Now it will fail to start the module and fixes the compilation
errors.
GenericI2C: use PIOS_INCLUDE_I2C to include whole code block
Merge pull request #561 from peabody124/i2c_include

GenericI2CSensor: do not fail compilation without PIOS_I2C_INCLUDE