sched: fix gcc 9 warning

gcc 9.1.1 warns about strncpy() which can lead to a missing null at the
end of the destination. Let's use strlcpy(), which doesn't have this
problem.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20190519122628.6226-2-nyh@scylladb.com>

include/osv/sched.hh

@@ -359,7 +359,7 @@ public:
             return *this;
         }
         attr& name(std::string n) {
-            strncpy(_name.data(), n.data(), sizeof(_name) - 1);
+            strlcpy(_name.data(), n.data(), sizeof(_name));
             return *this;
         }
     };