df_import_structures does not properly adduce minimum sizes of enums with negative values

[ab9rf]

df_import_structures identifies the building_type enum as requiring 32 bits even though it can fit in 16 (indeed probably 8) because the enum includes a value NONE with value -1. the enum imported into ghidra encodes this value as 0xffffffff instead of as -1 (which itself isn't necessarily wrong) and thus concluded from this that the enum required 4 bytes (which is)