feat(rak3112): add conditional macros for rakwireless-audio library support

[Ethan-chen1234-zy]

Add a variant-specific macro VARIANT_RAK3112 to pins_arduino.h to enable conditional compilation in Audio-related libraries (e.g., PDM, I2S configurations) that need to detect the RAK3112 board.

Changes
Added a header guard wrapper for RAK3112 variant at the beginning of pins_arduino.h:

#ifndef VARIANT_RAK3112
#define VARIANT_RAK3112
#endif
No other pins or definitions were modified, ensuring full backward compatibility.

Impact
This change does not affect existing functionality. It simply provides a standard way for libraries to check if the target board is RAK3112.

Testing
Compiled with Arduino IDE using board "RAK3112" and verified that the macro is correctly defined.

Tested with a custom Audio library that conditionally enables board-specific I2S pins and PDM settings; confirmed it compiles and runs without errors.

Additional Notes
This change is part of the ongoing effort to improve Audio support for RAK3112 (branch feature/rak3112-add-audio-support). The macro allows libraries to avoid hardcoding board checks based on other pins or assumptions.

[github-actions]

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