build: fix the build when using MonoDevelop instead of autotools

There were several errors of this type[1] which just required to
include missing references.

[1] Error CS0012: The type `Foo' is defined in an assembly that
is not referenced. Consider adding a reference to assembly
`Bar, Version=X.Y.Z.T, Culture=neutral, PublicKeyToken=null'
(CS0012) (Baz)