Add new file to CMakeLists to fix build

Author: stmontgomery

CMakeLists.txt

@@ -40,6 +40,7 @@ add_library(XCTest
   Sources/XCTest/Public/XCTestObservationCenter.swift
   Sources/XCTest/Public/XCTestCase+Performance.swift
   Sources/XCTest/Public/XCTAssert.swift
+  Sources/XCTest/Public/XCTSkip.swift
   Sources/XCTest/Public/Asynchronous/XCTNSNotificationExpectation.swift
   Sources/XCTest/Public/Asynchronous/XCTNSPredicateExpectation.swift
   Sources/XCTest/Public/Asynchronous/XCTWaiter+Validation.swift