Test case for incomplete types

The bcc rewriter segfaults when it encounters an incomplete type
(missing declaration or declaration after use) in a map declaration.

tests/python/test_clang.py

@@ -587,6 +587,16 @@ int bar(void) {
 int foo(struct pt_regs *ctx) {
     return bar();
 }
+"""
+        with self.assertRaises(Exception):
+            b = BPF(text=text)
+
+    def test_incomplete_type(self):
+        text = """
+BPF_HASH(drops, struct key_t);
+struct key_t {
+    u64 location;
+};
 """
         with self.assertRaises(Exception):
             b = BPF(text=text)