Define `pod_target_xcconfig` for PrivateDatabase

Summary:
We don't strictly need this since all the other subspecs that use this set up the header search path for ReactCommon anyway but do it just for correctness.
Closes https://github.com/facebook/react-native/pull/16665

Differential Revision: D6241796

Pulled By: hramos

fbshipit-source-id: 19e6363e570d38f06ea970395d68b9cea7fd02ae

React.podspec

@@ -120,6 +120,7 @@ Pod::Spec.new do |s|
   s.subspec "PrivateDatabase" do |ss|
     ss.source_files         = "ReactCommon/privatedata/*.{cpp,h}"
     ss.private_header_files = "ReactCommon/privatedata/*.h"
+    ss.pod_target_xcconfig  = { "HEADER_SEARCH_PATHS" => "\"$(PODS_TARGET_SRCROOT)/ReactCommon\"" }
   end
 
   s.subspec "cxxreact" do |ss|