fix: re-add mismerged stb include in CMake

While recovering the vcpkg-ports tree from an earlier commit via revert
some unrelated changes slipped through despite auditing the merge and
wasn't caught by an incremental rebuild.

Re-add the include directory for the vendored stb libraries.

CMakeLists.txt

@@ -150,6 +150,7 @@ target_link_libraries(imgui PUBLIC
 target_include_directories(SimpleGraphic
     PRIVATE
     ${CMAKE_CURRENT_SOURCE_DIR}/dep/glad/include
+    ${CMAKE_CURRENT_SOURCE_DIR}/dep/stb
     ${JPEG_INCLUDE_DIR}
 )