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.

gtk/gtksidebarrow.c

@@ -30,7 +30,7 @@
 #include "gtkspinner.h"
 
 #ifdef HAVE_CLOUDPROVIDERS
-#include <cloudproviders/cloudprovidersaccount.h>
+#include <cloudproviders.h>
 #endif
 
 struct _GtkSidebarRow