yasm: specify empty public header for assembly library

By specifying empty list in public header, unnecessary build dependency will be
removed from generated ninja files.

More detail in GN reference.
https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md#public

Bug: 578477
Change-Id: I2321ae53376de907d05a59f14d3578a11b2be584
Reviewed-on: https://chromium-review.googlesource.com/1037063
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#555026}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cab48032c45695c73b202af7d472a52808d551ca