Allow the last member of a struct to be an unsized/zero-length array.

This enables the C truck of allocating a dynamic amount of storage for
the struct in order to extend out the array to the desired length.

tests_errors/struct-unsized-array.ispc

@@ -1,4 +1,4 @@
-// Unsized arrays aren't allowed in struct definitions
+// Unsized arrays aren't allowed except for the last member in a struct definition.
 
 struct Foo {
     float a[];