vscode: Add hard-coded path to compile_commands.json

When run from Windows via WSL, VS Code just won't find the
`compile_commands.json` otherwise.

.vscode/c_cpp_properties.json

@@ -0,0 +1,11 @@
+{
+    "configurations": [
+        {
+            "name": "RIOT",
+            "cStandard": "c11",
+            "cppStandard": "c++17",
+            "compileCommands": "${workspaceFolder}/compile_commands.json"
+        }
+    ],
+    "version": 4
+}