fix: specify UTF-8 encoding when reading .csproj file on Windows

[YuroGod]

Problem

On Windows systems with non-UTF-8 locale (e.g. Simplified Chinese using GBK),
building with scons platform=windows fails with:

UnicodeDecodeError: 'gbk' codec can't decode byte 0xbf in position 2: illegal multibyte sequence:
  File "modules/gdsdecomp/build/mono_decomp.py", line 53:
    for line in csproj_file:

This happens because open() without an explicit encoding falls back to the
system default (GBK on Chinese Windows), but the .csproj file is UTF-8 encoded.

Fix

Explicitly set encoding="utf-8" in the open() call for .csproj files.

[nikitalita]

Thank you!