Add missing export of pluginlib boost disable flag

[devrite]

---

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#2429
Primary OS tested on	Ubuntu
Robotic platform tested on	none

---

Description of contribution in a few bullet points

• Used ament_export_definitions
• Veridied flag being applied by nav2_costmap_2d to gridmap_costmap_2d in foxy

Description of documentation updates required from your changes

None required. Should be transparent to the user.

---

Future work that may be required in bullet points

None. Rolling and galactic can just remove the flags (see #2431).

For Maintainers:

• Check that any new parameters added are updated in navigation.ros.org
• Check that any significant change is added to the migration guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists