BpfMap - fix a clang warning: misc-unconventional-assign-operator

//system/bpf/libbpf_android:libbpf_android_test clang-tidy BpfMapTest.cpp
system/bpf/libbpf_android/include/bpf/BpfMap.h:138:5: warning: operator=() should return 'BpfMap&' [misc-unconventional-assign-operator]
    void operator=(BpfMap<Key, Value>&& other) noexcept {
    ^

Test: builds without warnings
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I0bb0d42f58bba58c7b9220f062453d827438e85a

libbpf_android/include/bpf/BpfMap.h

@@ -133,10 +133,11 @@ class BpfMap {
         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;