[compiler-rt] Fix -Wcast-qual after D147005 (NFC)

/home/jiefu/llvm-project/compiler-rt/lib/lsan/lsan_allocator.cpp:161:18: error: cast from 'const void *' to 'void *' drops const qualifier [-Werror,-Wcast-qual]
  return (void *)beg;
                 ^
1 error generated.