Fix compilation error using CLANG

[cbi42]

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....

Test plan: make USE_CLANG=1 j=150 check with

rocksdb/build_tools/build_detect_platform

Lines 61 to 70 in 13f054f

	if [ -d /mnt/gvfs/third-party ]; then
	echo "NOTE: Using fbcode build" >&2
	FBCODE_BUILD="true"
	# If we're compiling with TSAN or shared lib, we need pic build
	PIC_BUILD=$COMPILE_WITH_TSAN
	if [ "$LIB_MODE" == "shared" ]; then
	PIC_BUILD=1
	fi
	source "$PWD/build_tools/fbcode_config_platform010.sh"
	else

commented out.

[facebook-github-bot]

@cbi42 has imported this pull request. If you are a Meta employee, you can view this in D80033441.

[facebook-github-bot]

@cbi42 merged this pull request in 496eeba.