Keep RTC API prototypes even when not enabled
Keep RTC API prototypes even when not enabled

Keep the prototypes in rtc_api.h even when DEVICE_RTC is not defined.
This allows devices that aren't fully compliant with the RTC API to
still use the header and prototypes.

mbedmicro/mbed,betzw/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,betzw/mbed-os,betzw/mbed-os,andcor02/mbed-os,betzw/mbed-os,mbedmicro/mbed,betzw/mbed-os