VMA_IMPLEMENTATION is defined in the build files, it doesn't need
to be redefined in vk_mem_alloc_wrapper.cpp.

BUG=angleproject:2162

Change-Id: Ie4eceedd2dd3051f8431c7c8c86403250e93f98b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2154538
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>