ebos: make its core headers self sufficient

this is part of the release maintainance. in this context "core
headers" means the ones which do not include the well model headers,
and only those which are concerned with non-exotic functionality,
e.g., the PolyhedralGrid and ALUGrid vanguards are not changed.

ebos/ebos.hh

@@ -25,6 +25,9 @@
  *
  * \brief The common settings for all ebos variants.
  */
+#ifndef EBOS_HH
+#define EBOS_HH
+
 #include "eclproblem.hh"
 
 #include <opm/autodiff/BlackoilWellModel.hpp>
@@ -133,3 +136,5 @@ public:
     using ParentType::EclProblem;
 };
 }
+
+#endif // EBOS_HH