fix a duplication warning causing by SPM function (#1532)

MJRefresh.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
     s.name         = 'MJRefresh'
-    s.version      = '3.7.1'
+    s.version      = '3.7.2'
     s.summary      = 'An easy way to use pull-to-refresh'
     s.homepage     = 'https://github.com/CoderMJLee/MJRefresh'
     s.license      = 'MIT'
@@ -8,6 +8,7 @@ Pod::Spec.new do |s|
     s.platform     = :ios, '9.0'
     s.source       = {:git => 'https://github.com/CoderMJLee/MJRefresh.git', :tag => s.version}
     s.source_files = 'MJRefresh/**/*.{h,m}'
+    s.exclude_files = 'MJRefresh/include/**'
     s.resource     = 'MJRefresh/MJRefresh.bundle'
     s.requires_arc = true
 end