libheif: disable aom for Rosetta case

[barracuda156]

Description

aom does not build for PPC, so it should be disabled for 10.6.x Rosetta too.

Type(s)

• [x] bugfix
• [ ] enhancement
• [ ] security fix

Tested on

macOS 10.6.8 Server Xcode 3.2.6

Verification

Have you

• [x] followed our Commit Message Guidelines?
• [x] squashed and minimized your commits?
• [x] checked that there aren't other open pull requests for the same change?
• [ ] referenced existing tickets on Trac with full URL?
• [x] checked your Portfile with port lint --nitpick?
• [ ] tried existing tests with sudo port test?
• [ ] tried a full install with sudo port -vst install?
• [ ] tested basic functionality of all binary files?

[macportsbot]

Notifying maintainers: @MarcusCalhoun-Lopez for port libheif.

[barracuda156]

@MarcusCalhoun-Lopez @mascguy Could someone approve this please? aom does not build for any PPC systems, including Rosetta. So dependency on it should be removed not only for <10, but for 10.6 + ppc as well.