Move Protobuf back to subspec and set requires_arc to false.

Setting requires_arc to false at the top level caused crashes when using the plugin.

Author: pauldemarco

ios/flutter_blue.podspec

@@ -18,8 +18,13 @@ Bluetooth Low Energy plugin for Flutter.
   s.dependency 'Flutter'
   s.platform = :ios, '8.0'
   s.framework = 'CoreBluetooth'
-  s.dependency 'Protobuf', '~> 3.11.4'
-  s.requires_arc = false
+
+  s.subspec "Protos" do |ss|
+    ss.source_files = "gen/*.pbobjc.{h,m}", "gen/**/*.pbobjc.{h,m}"
+    ss.header_mappings_dir = "gen"
+    ss.requires_arc = false
+    ss.dependency "Protobuf", '~> 3.11.4'
+  end
 
   # Flutter.framework does not contain a i386 slice. Only x86_64 simulators are supported.
   s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'VALID_ARCHS[sdk=iphonesimulator*]' => 'x86_64', 'GCC_PREPROCESSOR_DEFINITIONS' => '$(inherited) GPB_USE_PROTOBUF_FRAMEWORK_IMPORTS=1', }