update podspec to support static_framework

ios/flutter_ijkplayer.podspec

@@ -15,7 +15,7 @@ A new flutter plugin project.
   s.source_files = 'Classes/**/*' , 'IJKMediaFramework.framework'
   s.public_header_files = 'Classes/**/*.h'
   s.dependency 'Flutter'
-
+  s.static_framework = true
   s.ios.deployment_target = '8.0'
 
   # s.ios.vendored_frameworks = 'IJKMediaFramework.framework'