style(hex8): make new standard dependencies explicit

include/fesa/elements/hex8_element.h

@@ -12,7 +12,7 @@
 namespace fesa {
 namespace hex8_internal {
 struct LinearTangentCache;
-}
+}  // namespace hex8_internal
 
 /// @brief Owns all local quantities from one validated linear or TL trial.
 struct Hex8Response {

src/fesa/elements/hex8_element.cpp

@@ -1,6 +1,7 @@
 #include "fesa/elements/hex8_element.h"
 
 #include <cmath>
+#include <memory>
 #include <set>
 #include <stdexcept>
 #include <utility>

tests/unit/elements/hex8_high_low_test.cpp

@@ -2,8 +2,9 @@
 
 #include <gtest/gtest.h>
 
-#include <cmath>
+#include <array>
 #include <cfenv>
+#include <cmath>
 #include <cstdint>
 #include <cstring>
 #include <future>