Fix compilation error using CLANG (#13864)

Summary:
fix the following error showing up in continuous tests:
```
Makefile:186: Warning: Compiling in debug mode. Don't use the resulting binary in production
port/mmap.cc:46:15: error: first argument in call to 'memcpy' is a pointer to non-trivially copyable type 'rocksdb::MemMapping' [-Werror,-Wnontrivial-memcall]
   46 |   std::memcpy(this, &other, sizeof(*this));
      |               ^
port/mmap.cc:46:15: note: explicitly cast the pointer to silence this warning
   46 |   std::memcpy(this, &other, sizeof(*this));
      |               ^
      |               (void*)
1 error generated.
make: *** [Makefile:2580: port/mmap.o] Error 1
make: *** Waiting for unfinished jobs....
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/13864

Test Plan: `make USE_CLANG=1 j=150 check` with https://github.com/facebook/rocksdb/blob/13f054febb26100184eeefaac11877d735d45ac2/build_tools/build_detect_platform#L61-L70 commented out.

Reviewed By: mszeszko-meta

Differential Revision: D80033441

Pulled By: cbi42

fbshipit-source-id: b2330eea71fe28243236b75128ec6f3f1e971873

port/mmap.cc

@@ -43,7 +43,7 @@ MemMapping& MemMapping::operator=(MemMapping&& other) noexcept {
     return *this;
   }
   this->~MemMapping();
-  std::memcpy(this, &other, sizeof(*this));
+  std::memcpy(static_cast<void*>(this), &other, sizeof(*this));
   new (&other) MemMapping();
   return *this;
 }