emulators/emu64: extend patch to fix an issue with OSS

This update is require to unbreak the port under FreeBSD.
The issue was reported to upstream and the fix will be available in future
releases.

Background: In the upstream issue, it was revealed that the whole C64 emulation
depends on the audio stream (for best sync) and initializing the audio stream
failed when the SDL backend was OSS, because it needs a fragment size that is a
power of two. The attached patch is the minimal change to 5.0.18 from the
upstream commit that solved the problem.

PR: 245395
Submitted by: Ingo <hylaios@online.de>
Approved by: Felix Palmen <felix@palmen-it.de> (maintainer)
MFH: 2020Q2