Limit .gitignore of build directory to root

VSCode uses .gitignore to hide directories, without this change this includes `scripts/build`, making it impossible to open or search the directory inside VSCode.

Author: Spartan322

.gitignore

@@ -82,7 +82,7 @@ bin/*
 compile_commands.json
 
 # Out-of-source build directory
-build/
+/build/
 
 # ccls
 .ccls-cache