Bug C backend: Cannot import Union  #1914

@dataclass
class A:
    ax: i32
@ccall
@union
class D(Union):
    a: A = A()

semantic error: Only Subroutines, Functions, StructType, Variables and ExternalSymbol are currently supported in 'import'
  --> /Users/ronnuriel/git/copperhead/demo-apps/belex-direct-fragment/device/../mat_vec_device.py:19:1
   |
19 | from common import D