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.
Merge topic 'fix-include_directories-whitespace-handling' into next

a97240b Tests/IncludeDirectories: Avoid shared library with no symbols