Add library.json to the lib/YNF262 to treat it as a library and also modified the platformio.ini to use project folder variable

lib/YMF262/library.json

@@ -0,0 +1,10 @@
+{
+  "name": "YMF262",
+  "version": "0.1.0",
+  "description": "Bare-metal driver for Yamaha YMF262 OPL3",
+  "frameworks": "stm32cube",
+  "platforms": "ststm32",
+  "build": {
+    "includeDir": "include"
+  }
+}

platformio.ini

@@ -13,4 +13,4 @@ platform = ststm32
 board = nucleo_g474re
 framework = stm32cube
 lib_deps = ./lib/YMF262
-build_flags = -DUSE_HAL_DRIVER  -DSTM32G474xx -I lib/YMF262/include
+build_flags = -DUSE_HAL_DRIVER -DSTM32G474xx -I${PROJECT_DIR}/lib/YMF262/include