Set custom effect defines for LED Matrix

[zvecr]

Description

RGB Matrix has the following block https://github.com/qmk/qmk_firmware/blob/master/builddefs/common_features.mk#L563-L569, but the equivalent block is missing for LED Matrix.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• Fixes [Bug] LED_MATRIX_CUSTOM_KB and LED_MATRIX_CUSTOM_USER rules.mk variables don't trigger addition of necessary C/C++ defines #24743

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).