Added missing include in Equil.hpp

When running test in opm-parser, this never showed up as a problem since
Equil.hpp seems to be always included after <vector> is included (either
directly or by including other files). However, this can become a
problem when using opm-parser in other projects (eg, sunbeam)

opm/parser/eclipse/EclipseState/InitConfig/Equil.hpp

@@ -1,6 +1,8 @@
 #ifndef OPM_EQUIL_HPP
 #define OPM_EQUIL_HPP
 
+#include <vector>
+
 namespace Opm {
 
     class DeckKeyword;