Fix C++11 compatibility: use std::is_standard_layout<T>::value

[Chris-godz]

Please update static assertions to use the older std::is_standard_layout<T>::value syntax instead of the newer std::is_standard_layout_v<T>.

In file included from leveldb/util/comparator.cc:14:
leveldb/./util/no_destructor.h: In constructor ‘leveldb::NoDestructor<InstanceType>::NoDestructor(ConstructorArgTypes&& ...)’:
leveldb/./util/no_destructor.h:24:24: error: ‘is_standard_layout_v’ is not a member of ‘std’; did you mean ‘is_standard_layout’?
   24 |     static_assert(std::is_standard_layout_v<NoDestructor<InstanceType>>);
      |                        ^~~~~~~~~~~~~~~~~~~~
      |                        is_standard_layout
leveldb/./util/no_destructor.h:24:70: error: expected primary-expression before ‘>’ token
   24 |     static_assert(std::is_standard_layout_v<NoDestructor<InstanceType>>);
      |                                                                      ^~
leveldb/./util/no_destructor.h:24:72: error: expected primary-expression before ‘)’ token
   24 |     static_assert(std::is_standard_layout_v<NoDestructor<InstanceType>>);
      |                                                                        ^
make[2]: *** [CMakeFiles/leveldb.dir/build.make:468: CMakeFiles/leveldb.dir/util/comparator.cc.o] Fehler 1
make[1]: *** [CMakeFiles/Makefile2:183: CMakeFiles/leveldb.dir/all] Fehler 2