extmod/extmod.cmake: Add support to build btree module with CMake.

Signed-off-by: Damien George <damien@micropython.org>
diff --git a/ports/esp32/main/CMakeLists.txt b/ports/esp32/main/CMakeLists.txt
index 2bfc095..2223ecd 100644
--- a/ports/esp32/main/CMakeLists.txt
+++ b/ports/esp32/main/CMakeLists.txt
@@ -3,10 +3,10 @@
 
 # Include core source components.
 include(${MICROPY_DIR}/py/py.cmake)
-include(${MICROPY_DIR}/extmod/extmod.cmake)
 
 if(NOT CMAKE_BUILD_EARLY_EXPANSION)
     include(${MICROPY_DIR}/py/usermod.cmake)
+    include(${MICROPY_DIR}/extmod/extmod.cmake)
 endif()
 
 set(MICROPY_QSTRDEFS_PORT