[podspec] Define pod_target_xcconfig for PrivateDatabase

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.

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|