Fix: compile message_handler_lib with correct flags

Link core_driver as PUBLIC so the ETHOSU define is exported.

Change-Id: I8900c11cc1a17ec37b8666131079a5a49875c73f
diff --git a/applications/message_handler/lib/CMakeLists.txt b/applications/message_handler/lib/CMakeLists.txt
index 7e2826b..4ea4010 100644
--- a/applications/message_handler/lib/CMakeLists.txt
+++ b/applications/message_handler/lib/CMakeLists.txt
@@ -19,12 +19,14 @@
 add_library(message_handler_lib STATIC)
 
 target_include_directories(message_handler_lib PUBLIC include
-                           PRIVATE ${LINUX_DRIVER_STACK_PATH}/kernel)
+        PRIVATE ${LINUX_DRIVER_STACK_PATH}/kernel)
 
-target_link_libraries(message_handler_lib PUBLIC ethosu_mailbox
+target_link_libraries(message_handler_lib
+        PUBLIC
+        ethosu_mailbox
+        $<$<TARGET_EXISTS:ethosu_core_driver>:ethosu_core_driver>
         PRIVATE
         cmsis_device
-        $<$<TARGET_EXISTS:ethosu_core_driver>:ethosu_core_driver>
         freertos_kernel
         inference_process
         tflu)