How to add static library to pod dependencies

I am trying to distribute my pod (only C files) with Cocoapod. My code depends on a static library (.a), and I am able to correctly build it. But when launching 'pod lib lint' to validate the podspec it gives me the following error:

fatal error: 'openssl/aes.h' file not found

So it doesn't find the header files of the static library. This is my Podspec file

  spec.source_files  = "myPod/*.{c}", "myPod/include/*.h", "myPod/include/openssl/*.h"
  spec.preserve_paths = "myPod/*.{c}", "myPod/include/*.h", "myPod/include/openssl/*.h"
  spec.ios.vendored_libraries = "libs/libcrypto.a", "libs/libssl.a"
  spec.xcconfig = { "HEADER_SEARCH_PATHS" => "$(SRCROOT)/myPod/include", 'LIBRARY_SEARCH_PATHS' => "$(SRCROOT)/libs/**" }
  

The missing header files are the ones in myPod/include/openssl , so they should be found. How can I solve it?