osxbom: switch to lib-style dependency on libunwind

[cooljeanius]

In https://trac.macports.org/ticket/66250 it says that Macports-built libunwind can cause problems with other ports. This PR tweaks osxbom's dependency on libunwind to use a lib:-style dependency instead, so that the system libunwind can satisfy it instead.

Description

Type(s)

• enhancement

Tested on

macOS 15.5 24F74 x86_64
Xcode 16.4 16F6

Verification

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[ryandesign]

Why are the libmacho and libmacho-headers dependencies needed at all? I tried removing them from the port and was still able to build it.