Add Sendable annotations to SwiftVersion (#7527)

I fixed warning appears when building with Swift 6.

Author: coffmark

Sources/Basics/SwiftVersion.swift

@@ -16,7 +16,7 @@
 import TSCclibc
 #endif
 
-public struct SwiftVersion {
+public struct SwiftVersion: Sendable {
     /// The version number.
     public var version: (major: Int, minor: Int, patch: Int)