fix: compilation on x86_64-swift-linux-musl

[asevko]

When building for x86_64-swift-linux-musl

swift build --configuration release --swift-sdk x86_64-swift-linux-musl --product ObjectArchiveKit

the build failed with ambiguous use of 'ARFMAG':

/Users/asevko/work/forks/ObjectArchiveKit/Sources/ObjectArchiveKit/Model/ArchiveMemberHeader.swift:61:23: error: ambiguous use of 'ARFMAG'
 59 | extension ArchiveMemberHeader {
 60 |     public var isValid: Bool {
 61 |         guard fmag == ARFMAG,
    |                       `- error: ambiguous use of 'ARFMAG'
 62 |               Int(_size) != nil else {
 63 |             return false

ObjectArchiveKitC.ARFMAG:1:12: note: found this candidate in module 'ObjectArchiveKitC'
1 | public var ARFMAG: String { get }
  |            `- note: found this candidate in module 'ObjectArchiveKitC'

linux_ar.ARFMAG:1:12: note: found this candidate in module 'linux_ar'
1 | public var ARFMAG: String { get }
  |            `- note: found this candidate in module 'linux_ar'

That PR resolves that ambiguity via specifying usage of ARFMAG from ObjectArchiveKitC.

Note: the x86_64-swift-linux-musl toolchain is installed via swiftly; the active toolchain should also be managed by swiftly. In my case that was:

swiftly link 6.2.3

[p-x9]

Thank you