versioner: add extern "C" to tests.

Bug: https://github.com/android-ndk/ndk/issues/440
Test: python run_tests.py
Change-Id: Id893979146bc609a17bd1fa2a6bec6f10dfe4804
diff --git a/tools/versioner/tests/missing_arch/headers/foo.h b/tools/versioner/tests/missing_arch/headers/foo.h
index 176e7a3..5ba4794 100644
--- a/tools/versioner/tests/missing_arch/headers/foo.h
+++ b/tools/versioner/tests/missing_arch/headers/foo.h
@@ -1 +1,9 @@
-int foo();
\ No newline at end of file
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+int foo();
+
+#if defined(__cplusplus)
+}
+#endif