Add missing include for memcmp

memcmp is defined in cstring, yet it wasn't included in the files that
tried using it.

include/uuid.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstring>
 #include <string>
 #include <sstream>
 #include <iomanip>
@@ -869,4 +870,4 @@ namespace std
          return static_cast<result_type>(hasher(uuids::to_string(uuid)));
       }
    };
-}
+}

test/test_uuid.cpp

@@ -1,6 +1,7 @@
 #include "uuid.h"
 #include "catch.hpp"
 
+#include <cstring>
 #include <set>
 #include <unordered_set>
 #include <vector>
@@ -288,4 +289,4 @@ TEST_CASE("Test as_bytes", "[ops]")
       REQUIRE(memcmp(view.data(), arr.data(), arr.size()) == 0);
    }
 
-}
+}