[nat64] enable discovering NAT64 AIL prefix for OpenWRT (openthread#9441

)

According to the description of
openwrt/openwrt#11559, libanl is already
included in the musl c library, so we can turn this option on.

We enable OPENTHREAD_POSIX_CONFIG_NAT64_AIL_PREFIX_ENABLE on OpenWrt
platform for better experience.

src/posix/platform/CMakeLists.txt

@@ -150,7 +150,7 @@ target_link_libraries(openthread-posix
 )
 
 option(OT_TARGET_OPENWRT "enable openthread posix for OpenWRT" OFF)
-if(${CMAKE_SYSTEM_NAME} STREQUAL "Linux" AND NOT OT_TARGET_OPENWRT)
+if(${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
     target_compile_definitions(ot-posix-config
         INTERFACE "OPENTHREAD_POSIX_CONFIG_NAT64_AIL_PREFIX_ENABLE=1"
     )