include SD_MMC.h only when supported from MCU

[Jason2866]

Description:

fix warning in upcoming core

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.6
• The code change is tested and works with Tasmota core ESP32 V.3.0.1
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass