This fixes the bug where the UI_OT_eyedropper_colorband button is not clickable or active in the color ramp tools popup menu.
This fix tags the OT poll function by adding a subtype to the parent tools button by extending uiBut.
The custom_data had to be moved to the parent tools button as it seems UI_context_active_but_get(C) is not working on the tools popup menu buttons.
See the following for reference:
rBa6218502240499423d29157d3ca3dcb916909b64: Original bug when eyedropper was a button. This contains an alternative fix by @LazyDodo (LazyDodo)
D3965: Eyedropper button was moved from button to color ramp tools menu