GIT: Modify .gitignore to ignore wildcard for build directories

Helps if you have multiple build folders. There are other, dark ways to
hide extra build folders from git, but this is better.

See: https://github.com/citra-emu/citra/pull/6130

.gitignore

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 # Build directory
-[Bb]uild/
+[Bb]uild*/
 doc-build/
 
 # Generated source files