Set public headers to make all .h files public

Description

Export headers for SPM pakcage

Motivation and Context

Make header public (otherwise all class are in the module, and not exposed)

How Has This Been Tested?

Created a single view storyboard project, and added https://github.com/openium/MXParallaxHeader in Swift Packages of Xcode, then :

git diff TestMXP/ViewController.swift
diff --git a/TestMXP/ViewController.swift b/TestMXP/ViewController.swift
index f3c8b3b..754cdd0 100644
--- a/TestMXP/ViewController.swift
+++ b/TestMXP/ViewController.swift
@@ -7,12 +7,19 @@
 //

 import UIKit
+import MXParallaxHeader

 class ViewController: UIViewController {

     override func viewDidLoad() {
         super.viewDidLoad()
-        // Do any additional setup after loading the view.
+
+        let header = MXParallaxHeader()
+
+        let scrollView = UIScrollView()
+        scrollView.parallaxHeader = header
+
+        view.addSubview(scrollView)
     }

Screenshots (if appropriate):

Types of changes

[X] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

[ ] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly.

maxep / MXParallaxHeader

Set public headers to make all .h files public #122