Drop ignition-math6 from rviz_default_plugins link interface (#833)

None of the rviz_default_plugins ABI relies on consumers linking against
ignition-math6 directly.

The alternative is to add the ignition-math6 link directory to the
rviz_default_plugins link interface so that consumers can reliably find
the library.

Signed-off-by: Scott K Logan <logans@cottsay.net>

rviz_default_plugins/CMakeLists.txt

@@ -231,6 +231,9 @@ target_include_directories(rviz_default_plugins PUBLIC
 target_link_libraries(rviz_default_plugins PUBLIC
   rviz_ogre_vendor::OgreMain
   rviz_ogre_vendor::OgreOverlay
+)
+
+target_link_libraries(rviz_default_plugins PRIVATE
   ignition-math6
 )