Structure symbol fix

[adrianpbrown]

Fixed issue with structures defining symbols without prefix as well meaning two structures could not contain the same name element

[simonowen]

I went to add a test for the described behaviour but it's passing in the old version, before your PR changes. Shouldn't this code fail to assemble?

Struct1:  struct
XPos:     rs  1
YPos:     rs  1
          ends

Struct2:  struct
YPos:     rs  2
XPos:     rs  2
          ends

          assert(Struct1.sizeof == 2)
          assert(Struct2.sizeof == 4)

          assert(Struct1.XPos == 0)
          assert(Struct1.YPos == 1)

          assert(Struct2.YPos == 0)
          assert(Struct2.XPos == 2)

[adrianpbrown]

Strange, i got an error with that sort of thing, however i did have a fix for another issue as well at the time, cant recall exactly the repro but it would define Struct1 and Struct2 with the address of PC when it found them which it shouldnt have done. (The Struct1 and Struct2 should never have been marked as symbols on their own)