make the PETSc code compile even if PETSc is not available

if PETSc is not available, the .cpp file will compile fine because it
will be reduced to be empty, but trying to include
LinearSolverPetsc.hpp in this case will result in an error.

opm/core/linalg/LinearSolverPetsc.cpp

@@ -19,6 +19,9 @@
 */
 
 #include "config.h"
+
+#if HAVE_PETSC
+
 #include <cstring>
 #include <opm/core/linalg/LinearSolverPetsc.hpp>
 #include <unordered_map>
@@ -287,3 +290,4 @@ namespace{
 
 } // namespace Opm
 
+#endif // HAVE_PETSC

opm/core/linalg/LinearSolverPetsc.hpp

@@ -20,6 +20,11 @@
 
 #ifndef OPM_LINEARSOLVERPETSC_HEADER_INCLUDED
 #define OPM_LINEARSOLVERPETSC_HEADER_INCLUDED
+
+#if !HAVE_PETSC
+#error "LinearSolverPetsc.hpp included, but the PETSc libraries are not available!"
+#endif
+
 #include <opm/core/linalg/LinearSolverInterface.hpp>
 #include <opm/core/utility/parameters/ParameterGroup.hpp>
 #include <string>