Merge #68: Add missed #include <string>

ed6c8fc Add missed `#include <string>` (Hennadii Stepanov)

Pull request description:

  Required for `std::stoll`.

  Otherwise, MSVC compiler fails to build the `test.exe` binary with errors as follows:
  ```
  C:\Users\hebasto\minisketch\src\test.cpp(276,41): error C2039: 'stoll': is not a member of 'std'
  C:\Users\hebasto\minisketch\src\test.cpp(276,46): error C3861: 'stoll': identifier not found
  ```

ACKs for top commit:
  sipa:
    ACK ed6c8fc

Tree-SHA512: 7ef4f7a852c37aa8d349ecef8c57f2f0c0a1aa6d038ba677687deca92d2541144ebc5279a7fb1f9cac7fb9b3ef258dcf678ae3230cb801b00eccc7a231a2f82c

Author: sipa

src/test.cpp

@@ -9,6 +9,7 @@
 #include <limits>
 #include <random>
 #include <stdexcept>
+#include <string>
 #include <vector>
 
 #include "../include/minisketch.h"