UI: Use Eyedropper Cursor with Sample Weight Tool

While using the Sample Weight tool in Weight Paint mode, user eyedropper mouse cursor.

Differential Revision: https://developer.blender.org/D9431

Reviewed by Pablo Vazquez

release/scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -1526,6 +1526,7 @@ class _defs_weight_paint:
             idname="builtin.sample_weight",
             label="Sample Weight",
             icon="ops.paint.weight_sample",
+            cursor='EYEDROPPER',
             widget=None,
             keymap=(),
             draw_settings=draw_settings
@@ -1537,6 +1538,7 @@ class _defs_weight_paint:
             idname="builtin.sample_vertex_group",
             label="Sample Vertex Group",
             icon="ops.paint.weight_sample_group",
+            cursor='EYEDROPPER',
             widget=None,
             keymap=(),
         )