Fixed the mbedtls installation cmake: now private headers, which are used in the installation, are included in it too

Signed-off-by: Anton Matkin <anton.matkin@arm.com>
diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index 755efed..9ea17af 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -7,6 +7,12 @@
     install(FILES ${headers}
         DESTINATION include/mbedtls
         PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)
+
+    file(GLOB private_headers "mbedtls/private/*.h")
+
+    install(FILES ${private_headers}
+        DESTINATION include/mbedtls/private
+        PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)
 endif(INSTALL_MBEDTLS_HEADERS)
 
 # Make mbedtls_config.h available in an out-of-source build. ssl-opt.sh requires it.