[NFC][gn build] Inclusive language: replace master with main in sync_source_lists_from_cmake.py

[NFC] As part of using inclusive language within the llvm project and to
match the renamed master branch, this patch replaces master with main in
sync_source_lists_from_cmake.py.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D113926