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/version_mismatch/headers/foo.h b/tools/versioner/tests/version_mismatch/headers/foo.h
index 4604092..ea35f36 100644
--- a/tools/versioner/tests/version_mismatch/headers/foo.h
+++ b/tools/versioner/tests/version_mismatch/headers/foo.h
@@ -1,5 +1,13 @@
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 #if __ANDROID_API__ <= 9
 int foo() __INTRODUCED_IN(9);
 #else
 int foo() __INTRODUCED_IN(10);
 #endif
+
+#if defined(__cplusplus)
+}
+#endif