Made C's sizeof operator work with initialized void variables.

Added regression tests that check cc65's handling of void variables.

test/err/void-empty.c

@@ -0,0 +1,9 @@
+/*
+  !!DESCRIPTION!! Uninitialized void variables
+  !!ORIGIN!!      cc65 regression tests
+  !!LICENCE!!     Public Domain
+  !!AUTHOR!!      Greg King
+*/
+
+void test;
+const void list;