fix unit tests build

test/test_file.cpp

@@ -32,6 +32,7 @@ POSSIBILITY OF SUCH DAMAGE.
 
 #include "libtorrent/file.hpp"
 #include "test.hpp"
+#include <string.h> // for strcmp
 #include <vector>
 #include <set>
 

test/test_string.cpp

@@ -34,6 +34,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/escape_string.hpp"
 #include "libtorrent/string_util.hpp"
 #include <iostream>
+#include <string.h> // for strcmp
 
 using namespace libtorrent;