stm32_common: i2c_2: fix for -Wunused-function
stm32_common: i2c_2: fix for -Wunused-function

This came up when compiling an application for a STM32-based board
with LLVM/clang. The function is not used if I²C is not provided.

BytesGalore/RIOT,mtausig/RIOT,gebart/RIOT,ant9000/RIOT,mfrey/RIOT,biboc/RIOT,kbumsik/RIOT,smlng/RIOT,x3ro/RIOT,cladmi/RIOT,lazytech-org/RIOT,basilfx/RIOT,lazytech-org/RIOT,aeneby/RIOT,kbumsik/RIOT,avmelnikoff/RIOT,aeneby/RIOT,RIOT-OS/RIOT,basilfx/RIOT,A-Paul/RIOT,x3ro/RIOT,miri64/RIOT,josephnoir/RIOT,avmelnikoff/RIOT,x3ro/RIOT,avmelnikoff/RIOT,mtausig/RIOT,ant9000/RIOT,x3ro/RIOT,RIOT-OS/RIOT,toonst/RIOT,aeneby/RIOT,RIOT-OS/RIOT,avmelnikoff/RIOT,smlng/RIOT,cladmi/RIOT,OlegHahm/RIOT,rfuentess/RIOT,kbumsik/RIOT,ant9000/RIOT,biboc/RIOT,authmillenon/RIOT,mfrey/RIOT,jasonatran/RIOT,OlegHahm/RIOT,gebart/RIOT,josephnoir/RIOT,authmillenon/RIOT,ant9000/RIOT,A-Paul/RIOT,ant9000/RIOT,yogo1212/RIOT,A-Paul/RIOT,rfuentess/RIOT,gebart/RIOT,aeneby/RIOT,rfuentess/RIOT,yogo1212/RIOT,toonst/RIOT,lazytech-org/RIOT,kaspar030/RIOT,josephnoir/RIOT,yogo1212/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,mtausig/RIOT,josephnoir/RIOT,OTAkeys/RIOT,kbumsik/RIOT,mtausig/RIOT,kaspar030/RIOT,OTAkeys/RIOT,smlng/RIOT,A-Paul/RIOT,kbumsik/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,biboc/RIOT,yogo1212/RIOT,OTAkeys/RIOT,toonst/RIOT,yogo1212/RIOT,yogo1212/RIOT,gebart/RIOT,cladmi/RIOT,biboc/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,mfrey/RIOT,kYc0o/RIOT,mtausig/RIOT,OTAkeys/RIOT,gebart/RIOT,kYc0o/RIOT,basilfx/RIOT,josephnoir/RIOT,kYc0o/RIOT,rfuentess/RIOT,miri64/RIOT,cladmi/RIOT,lazytech-org/RIOT,basilfx/RIOT,kYc0o/RIOT,aeneby/RIOT,jasonatran/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,smlng/RIOT,kaspar030/RIOT,smlng/RIOT,OlegHahm/RIOT,mfrey/RIOT,mfrey/RIOT,cladmi/RIOT,biboc/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,BytesGalore/RIOT,basilfx/RIOT,A-Paul/RIOT,miri64/RIOT,kaspar030/RIOT,rfuentess/RIOT,authmillenon/RIOT,avmelnikoff/RIOT,BytesGalore/RIOT,BytesGalore/RIOT,jasonatran/RIOT,toonst/RIOT,x3ro/RIOT,authmillenon/RIOT,toonst/RIOT