Use the appropriate include for libcloudproviders

Use the appropriate include for libcloudproviders

Libcloudproviders has a single header entry point, so we shouldn't
include a sub-header.

Additionally, the include path provided by the pkg-config file is:

  -I${includedir}/cloudproviders

So the include directive should be:

  #include <cloudproviders.h>

The fact that it worked until now was an accident caused by the blanket:

  -I${includedir}

we get for free; it broke the build when using libcloudproviders as a
subproject.