ARROW-1258: [C++] Suppress Clang dlmalloc compiler warnings

[wesm]

cc @pcmoritz

[pcmoritz]

Thank a lot for this! The changes look good to me. On both my local mac and linux there are no warnings any more, on travis linux there is still a few of these:

In file included from /home/travis/build/apache/arrow/cpp/src/plasma/malloc.cc:45:0:
/home/travis/build/apache/arrow/cpp/src/plasma/thirdparty/dlmalloc.c: In function ‘void* prepend_alloc(mstate, char*, char*, size_t)’:
/home/travis/build/apache/arrow/cpp/src/plasma/thirdparty/dlmalloc.c:3982:892: warning: conversion to ‘unsigned int’ from ‘size_t {aka long unsigned int}’ may alter its value [-Wconversion]
     insert_chunk(m, q, qsize);
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https://travis-ci.org/apache/arrow/jobs/257422813