[spinel] fix use of OT header file in spinel.c (#5141)
[spinel] fix use of OT header file in spinel.c (#5141)

`spinel.h` and `spinel.c` are soruce files that may be used by host
side drivers and therefore the souurce code itself should not use any
of OT header files. Spienl file provide `SPINEL_PLATFORM_HEADER` to
allow platform to add their own includes if they want to.
bukepo/openthread,chshu/openthread,librasungirl/openthread,abtink/openthread,chshu/openthread,srickardti/openthread,bukepo/openthread,lanyuwen/openthread,abtink/openthread,chshu/openthread,bukepo/openthread,chshu/openthread,jwhui/openthread,librasungirl/openthread,srickardti/openthread,lanyuwen/openthread,openthread/openthread,lanyuwen/openthread,openthread/openthread,jwhui/openthread,librasungirl/openthread,abtink/openthread,chshu/openthread,openthread/openthread,lanyuwen/openthread,srickardti/openthread,bukepo/openthread,openthread/openthread,abtink/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,jwhui/openthread,srickardti/openthread,lanyuwen/openthread,jwhui/openthread