update themis podspec to use 'header mappings dir' instead of 'subspec'
update themis podspec to use 'header mappings dir' instead of 'subspec'

cossacklabs/themis,cossacklabs/themis,storojs72/themis,storojs72/themis,cossacklabs/themis,Lagovas/themis,mnaza/themis,storojs72/themis,mnaza/themis,cossacklabs/themis,cossacklabs/themis,storojs72/themis,Lagovas/themis,storojs72/themis,mnaza/themis,Lagovas/themis,Lagovas/themis,mnaza/themis,Lagovas/themis,storojs72/themis,cossacklabs/themis,mnaza/themis,storojs72/themis,Lagovas/themis,storojs72/themis,mnaza/themis,cossacklabs/themis,mnaza/themis,cossacklabs/themis,cossacklabs/themis,mnaza/themis,Lagovas/themis,Lagovas/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,cossacklabs/themis,mnaza/themis,storojs72/themis,storojs72/themis,Lagovas/themis