Use /utf-8 compiler option to fix MSVC C4819 warning.

[zufuliu]

https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4819?view=msvc-170

The file contains a character that cannot be represented in the current code page (number). Save the file in Unicode format to prevent data loss.

https://learn.microsoft.com/en-us/cpp/build/reference/utf-8-set-source-and-executable-character-sets-to-utf-8?view=msvc-170

Specifies both the source character set and the execution character set as UTF-8.

PR #1915 added UTF-8 manifest, I think use /utf-8 would be a nature fix for C4819 warning (on src/depfile_parser.cc), instead of just suppress it.

cc @jhasse