Set public headers to make all .h files public

[kenji21]

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

• 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.

[kenji21]

For reference, issue : #116 and my previous PR #121