[Metal] Added "default.metallib" to .gitignore.

These files can show up during testing when compiling local Metal shaders.
Only keep built-in *.metallib files under Metal/Shader/Builtin/.

.gitignore

@@ -14,6 +14,7 @@ Screenshot.*
 Capture.*
 *.Results.png
 LLGL.trace.json
+default.metallib
 
 # Output files from individual tests (Testbed is handled separately)
 tests/Output
@@ -27,4 +28,4 @@ tests/Output
 *.cap
 
 # VS Code folder
-.vscode
+.vscode