BpfMap - fix a clang warning: misc-unconventional-assign-operator am: 8ed7c0f9b9 am: cf1d21c5d8

Original change: https://android-review.googlesource.com/c/platform/system/bpf/+/1552943

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I8992b3bb71030d37751ad3e101f07841bd2b2bd0
diff --git a/libbpf_android/include/bpf/BpfMap.h b/libbpf_android/include/bpf/BpfMap.h
index c42801a..bdffc0f 100644
--- a/libbpf_android/include/bpf/BpfMap.h
+++ b/libbpf_android/include/bpf/BpfMap.h
@@ -133,10 +133,11 @@
         return *this;
     }
 
-    // Move constructor
-    void operator=(BpfMap<Key, Value>&& other) noexcept {
+    // Move assignment operator
+    BpfMap<Key, Value>& operator=(BpfMap<Key, Value>&& other) noexcept {
         mMapFd = std::move(other.mMapFd);
         other.reset(-1);
+        return *this;
     }
 
     void reset(base::unique_fd fd) = delete;