CMake: psa: Enable installing of headers

When installing via CMake, also install PSA headers so that PSA APIs can be
more easily used from the installed library.
diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index 1b581a5..67c66c8 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -3,11 +3,16 @@
 if(INSTALL_MBEDTLS_HEADERS)
 
     file(GLOB headers "mbedtls/*.h")
+    file(GLOB psa_headers "psa/*.h")
 
     install(FILES ${headers}
         DESTINATION include/mbedtls
         PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)
 
+    install(FILES ${psa_headers}
+        DESTINATION include/psa
+        PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)
+
 endif(INSTALL_MBEDTLS_HEADERS)
 
 # Make config.h available in an out-of-source build. ssl-opt.sh requires it.