Fixes build by moving the prepare_command back up to the top level

gRPC.podspec

@@ -16,7 +16,9 @@ Pod::Spec.new do |s|
     rs.summary  = 'Reactive Extensions library for iOS.'
     rs.authors  = { 'Jorge Canizales' => 'jcanizales@google.com' }
 
-    rs.source_files = 'src/objective-c/RxLibrary/*.{h,m}', 'src/objective-c/RxLibrary/transformations/*.{h,m}', 'src/objective-c/RxLibrary/private/*.{h,m}'
+    rs.source_files = 'src/objective-c/RxLibrary/*.{h,m}',
+                      'src/objective-c/RxLibrary/transformations/*.{h,m}',
+                      'src/objective-c/RxLibrary/private/*.{h,m}'
     rs.private_header_files = 'src/objective-c/RxLibrary/private/*.h'
   end
 
@@ -36,10 +38,13 @@ Pod::Spec.new do |s|
     cs.requires_arc = false
     cs.libraries = 'z'
     cs.dependency 'OpenSSL', '~> 1.0.200'
+  end
 
   # This is a workaround for Cocoapods Issue #1437.
   # It renames time.h and string.h to grpc_time.h and grpc_string.h.
-    cs.prepare_command = <<-CMD
+  # It needs to be here (top-level) instead of in the C-Core subspec because Cocoapods doesn't run
+  # prepare_command's of subspecs.
+  s.prepare_command = <<-CMD
     DIR_TIME="grpc/support"
     BAD_TIME="$DIR_TIME/time.h"
     GOOD_TIME="$DIR_TIME/grpc_time.h"
@@ -58,13 +63,13 @@ Pod::Spec.new do |s|
       mv "$BAD_STRING" "$GOOD_STRING"
     fi
   CMD
-  end
 
   s.subspec 'GRPCClient' do |gs|
     gs.summary = 'Objective-C wrapper around the core gRPC library.'
     gs.authors  = { 'Jorge Canizales' => 'jcanizales@google.com' }
 
-    gs.source_files = 'src/objective-c/GRPCClient/*.{h,m}', 'src/objective-c/GRPCClient/private/*.{h,m}'
+    gs.source_files = 'src/objective-c/GRPCClient/*.{h,m}',
+                      'src/objective-c/GRPCClient/private/*.{h,m}'
     gs.private_header_files = 'src/objective-c/GRPCClient/private/*.h'
 
     gs.dependency 'gRPC/C-Core'