emulators/virtualbox-ose: Avoid overwriting slave ports OPTIONS_EXCLUDE
To disable the PULSEAUDIO option I used the OPTIONS_EXCLUDE variable
in commit 416b34d584e268.
Turns out this overwrites OPTIONS_EXCLUDE from slave ports. I'm
fixing this by only adding the PULSEAUDIO exclusion to the existing
EXCLUDE variable if defined.
NOTE: While it would be enough to bump PORTREVISION in the slave
port, I'm bumping PORTREVISION on master port to avoid adding
variables that are easily forgotten in future changes and could
cause even more breakage.
PR: 258256
Pointy Hat: myself