fixed db_stress.cc build error (#5307)

Summary:
when building this file using Xcode 10.2.1 in MacOSX10.14, the compiler report this error:
`
rocksdb/tools/db_stress.cc:3613:33: error: implicit instantiation of
      undefined template 'std::__1::array<std::__1::basic_string<char>, 10>'
    std::array<std::string, 10> keys = {"0", "1", "2", "3", "4", "5", "6", "7", "8", "9"};
/usr/include/c++/v1/__tuple:223:64: note:
      template is declared here
template <class _Tp, size_t _Size> struct _LIBCPP_TEMPLATE_VIS array;
                                                               ^
1 error generated.
`
if including array, this error will be fixed.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5307

Differential Revision: D15475217

Pulled By: sagar0

fbshipit-source-id: b04a7658c2ca2573157028863b3a80f5ab52b9de

tools/db_stress.cc

@@ -38,6 +38,7 @@ int main() {
 #include <stdlib.h>
 #include <sys/types.h>
 #include <algorithm>
+#include <array>
 #include <chrono>
 #include <exception>
 #include <queue>