fix build error on case-insensitive filesystems

compilerAst/src/prog8/ast/antlr/Antlr2Kotlin.kt

@@ -401,7 +401,7 @@ private fun Assign_targetContext.toAst() : AssignTarget {
             AssignTarget(null, arrayindexed, null, null, false, toPosition())
         }
         is VoidTargetContext -> {
-            AssignTarget(null, null, null, null, true, voidtarget().toPosition())
+            AssignTarget(null, null, null, null, true, this.toPosition())
         }
         else -> throw FatalAstException("weird assign target node $this")
     }

parser/src/main/antlr/Prog8ANTLR.g4

@@ -174,10 +174,9 @@ assign_target:
     scoped_identifier               #IdentifierTarget
     | arrayindexed                  #ArrayindexedTarget
     | directmemory                  #MemoryTarget
-    | voidtarget                    #VoidTarget
+    | VOID                          #VoidTarget
     ;
 
-voidtarget : VOID ;
 
 multi_assign_target:
     assign_target (',' assign_target)+ ;