extmod/extmod.cmake: Only include modbtree in build if it's enabled.

Following how it's done in extmod.mk.

Signed-off-by: Damien George <damien@micropython.org>

extmod/extmod.cmake

@@ -14,7 +14,6 @@ set(MICROPY_SOURCE_EXTMOD
     ${MICROPY_EXTMOD_DIR}/machine_signal.c
     ${MICROPY_EXTMOD_DIR}/machine_spi.c
     ${MICROPY_EXTMOD_DIR}/modbluetooth.c
-    ${MICROPY_EXTMOD_DIR}/modbtree.c
     ${MICROPY_EXTMOD_DIR}/modframebuf.c
     ${MICROPY_EXTMOD_DIR}/modlwip.c
     ${MICROPY_EXTMOD_DIR}/modnetwork.c
@@ -97,6 +96,10 @@ if(MICROPY_PY_BTREE)
         __DBINTERFACE_PRIVATE=1
         "virt_fd_t=void*"
     )
+
+    list(APPEND MICROPY_SOURCE_EXTMOD
+        ${MICROPY_EXTMOD_DIR}/modbtree.c
+    )
 endif()
 
 # Library for mbedtls