import_ghidra - bitfields should not import enum items with values less than 1 #78

When importing bitfields which contain enums, any enum entries equal to zero should be skipped, since otherwise it causes Ghidra to issue warnings about non-unique values.

There's no point in importing those, anyways, because we always create a none_of_<bitfield> entry equal to zero, and the decompiler is never going to be smart enough to know which of the others to select.

We should also never import enum entries with negative values - for example, tree_leaf_graphics_flag imports with an entry tree_leaf_graphics_flag_tile_NONE equal to -0x100.