Upstream has released updates that appears to apply and compile correctly.
This update has not been tested by PaperMC and as with ANY update, please do your own testing
Bukkit Changes:
d58803c2 SPIGOT-4597: Make BlockDropItemEvent use a list
d450fdf1 Add note about Maven repository usage to pom.xml
CraftBukkit Changes:
ed274c51 SPIGOT-4597: Make BlockDropItemEvent use a list
1734f378 SPIGOT-4598: Shift click on custom workbench inventory causes crash
a65b73ad Add note about Maven repository usage to pom.xml