Fix Arduino.h not found when building vendored GxEPD2 sources

The vendored sources are registered as their own ESP-IDF component,
so the arduino_compat include path added via cg.add_build_flag (which
only reaches the main app component) never reached it.

components/gxepd2/vendor/CMakeLists.txt

@@ -14,5 +14,6 @@ endif()
 idf_component_register(
     SRCS ${GXEPD2_SOURCES}
     INCLUDE_DIRS "src"
+    PRIV_INCLUDE_DIRS "../arduino_compat"
     PRIV_REQUIRES driver
 )