pipe: add missing return in pipe's operator='s

pipe_reader's and pipe_writer's operator= were missing
their return statements, which are now added.
Message-Id: <ba32476100e2cb4aeaa57ab3242f07c020b277df.1659012359.git.sarna@scylladb.com>

include/seastar/core/pipe.hh

@@ -182,6 +182,7 @@ public:
     }
     pipe_reader& operator=(pipe_reader&& other) noexcept {
         std::swap(_bufp, other._bufp);
+        return *this;
     }
 };
 
@@ -221,6 +222,7 @@ public:
     }
     pipe_writer& operator=(pipe_writer&& other) noexcept {
         std::swap(_bufp, other._bufp);
+        return *this;
     }
 };