Fix async_test.cc compilation

[misty-fungus]

Hi, had problems compiling tests.
Here's small hotfix.

General worflow:

git clone ...
mkdir build && cd build
cmake ../pistache
make

[ 86%] Linking CXX executable hello_server
/home/misty_fungus/workspace/pistache/tests/async_test.cc:382:10: error: ‘random_device’ in namespace ‘std’ does not name a type
     std::random_device rd;
          ^~~~~~~~~~~~~
[ 86%] Built target hello_server

[oktal]

Hi, thanks for the fix