Use memcmp() from namespace std

Don't rely on clients using

    #include <string.h>

before including cmp.hpp .

opm/common/utility/numeric/cmp.hpp

@@ -21,6 +21,7 @@
 #define COMMON_UTIL_NUMERIC_CMP
 
 #include <cstddef>
+#include <cstring>
 #include <vector>
 #include <type_traits>
 #include <cmath>
@@ -109,7 +110,7 @@ namespace Opm {
 
         template<typename T>
         bool array_equal(const T* p1, const T* p2, size_t num_elements, T abs_eps, T rel_eps) {
-            if (memcmp(p1 , p2 , num_elements * sizeof * p1) == 0)
+            if (std::memcmp(p1 , p2 , num_elements * sizeof * p1) == 0)
                 return true;
             else {
                 size_t index;