infohash: constexpr HexMap constructor

include/opendht/infohash.h

@@ -345,8 +345,8 @@ Hash<N>::getRandom(Rd& rdev)
 }
 
 struct alignas(std::max_align_t) HexMap : public std::array<std::array<char, 2>, 256> {
-    HexMap() {
-        for (size_t i=0; i<size(); i++) {
+    constexpr HexMap() : std::array<std::array<char, 2>, 256>() {
+        for (size_t i = 0; i < size(); i++) {
             auto& e = (*this)[i];
             e[0] = hex_digits[(i >> 4) & 0x0F];
             e[1] = hex_digits[i & 0x0F];