Add GPB_USE_PROTOBUF_FRAMEWORK_IMPORTS=1 to podspec

Author: pauldemarco

ios/flutter_blue.podspec

@@ -27,6 +27,6 @@ Bluetooth Low Energy plugin for Flutter.
   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', }
+  s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'VALID_ARCHS[sdk=iphonesimulator*]' => 'x86_64', 'GCC_PREPROCESSOR_DEFINITIONS' => '$(inherited) GPB_USE_PROTOBUF_FRAMEWORK_IMPORTS=1', }
 
-end
+end