Tests/IncludeDirectories: Avoid shared library with no symbols

The parent commit added a empty_entry_test shared library to the
IncludeDirectories test.  Some toolchains fail to create a shared
library with no user-defined symbols, so provide a dummy symbol.

Tests/IncludeDirectories/empty.cpp

@@ -1 +1,4 @@
-// No content
+#ifdef _WIN32
+__declspec(dllexport)
+#endif
+int empty() { return 0; }