cmake(bugfix):make sure the extra lib only added once

avoid duplicate definitions of target when
`nuttx_add_extra_library` is called between different models at the same time

Signed-off-by: xuxin19 <xuxin19@xiaomi.com>

cmake/nuttx_add_library.cmake

@@ -201,8 +201,10 @@ function(nuttx_add_extra_library)
     # define the target name of the extra library
     string(REGEX REPLACE "[^a-zA-Z0-9]" "_" extra_target "${extra_lib}")
     # set the absolute path of the library for the import target
+    if(NOT TARGET ${extra_target})
       nuttx_library_import(${extra_target} ${extra_lib})
       set_property(GLOBAL APPEND PROPERTY NUTTX_EXTRA_LIBRARIES ${extra_target})
+    endif()
   endforeach()
 endfunction()