boot_serial: Fix echo command code no longer compiling

Fixing missed update from cddl to zcbor.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/zephyr/CMakeLists.txt b/boot/zephyr/CMakeLists.txt
index db9067a..6070cd5 100644
--- a/boot/zephyr/CMakeLists.txt
+++ b/boot/zephyr/CMakeLists.txt
@@ -256,6 +256,8 @@
   zephyr_sources(${BOOT_DIR}/boot_serial/src/zcbor_encode.c)
   zephyr_sources(${BOOT_DIR}/boot_serial/src/zcbor_common.c)
 
+  zephyr_sources_ifdef(CONFIG_BOOT_MGMT_ECHO ${BOOT_DIR}/boot_serial/src/serial_recovery_echo.c)
+
   zephyr_include_directories(${BOOT_DIR}/bootutil/include)
   zephyr_include_directories(${BOOT_DIR}/boot_serial/include)
   zephyr_include_directories(include)