Fix the Swifter.podspec path to the source files

[welsonpan360]

After forking the repository, pointing my Podfile at the fork, and running pod install, I found that my Pods/Swifter directory was empty.

Since the source files now exist in XCode/Sources/, the podspec should be updated.

[swifter-bot]

	2 Warnings
⚠️	It seems like you’ve added new tests to the library. If that’s the case, please update the XCTestManifests.swift file running in your terminal the command swift test --generate-linuxmain.
⚠️	If you ran the command swift test --generate-linuxmain in your terminal, please remove the line testCase(IOSafetyTests.__allTests__IOSafetyTests), from public func __allTests() -> [XCTestCaseEntry] in the bottom of the file. For more reference see #366.

	1 Message
📖	Hey, @welsonpan 👋.

Generated by 🚫 Danger

[Vkt0r]

This looks good to me! Thanks @welsonpan