3rdparty: Fix inclusion order of CMakeLists.txt

This is so that third-party modules pick up the INSTALL_MBEDTLS_HEADERS variable.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 57b4b3a..6292c52 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -181,8 +181,6 @@
     set(LIB_INSTALL_DIR lib)
 endif()
 
-include_directories(include/)
-
 if(ENABLE_ZLIB_SUPPORT)
     find_package(ZLIB)