[spinel] Remove `#include <openthread/config.h> from `spinel.c` (#1951)
[spinel] Remove `#include <openthread/config.h> from `spinel.c` (#1951)

The `<openthread/config.h>` header file is already included from
OpenThread specific platform header file `spinel_platform.h`.
pvanhorn/openthread,fbsder/openthread,pvanhorn/openthread,LeZhang2016/openthread,erja-gp/openthread,fbsder/openthread,JakubBrachTieto/openthread,xiaom-GitHub/openthread,abtink/openthread,srickardti/openthread,aeliot/openthread,turon/openthread,mszczodrak/openthread,jwhui/openthread,xiaom-GitHub/openthread,gandreello/openthread,LeZhang2016/openthread,chshu/openthread,jwhui/openthread,LeZhang2016/openthread,xiaom-GitHub/openthread,gandreello/openthread,bukepo/openthread,erja-gp/openthread,vaas-krish/openthread,chshu/openthread,jwhui/openthread,librasungirl/openthread,bukepo/openthread,mszczodrak/openthread,aeliot/openthread,mszczodrak/openthread,georgecpr/openthread,georgecpr/openthread,lanyuwen/openthread,georgecpr/openthread,LeZhang2016/openthread,LeZhang2016/openthread,georgecpr/openthread,mszczodrak/openthread,gandreello/openthread,aeliot/openthread,lanyuwen/openthread,vaas-krish/openthread,librasungirl/openthread,openthread/openthread,erja-gp/openthread,abtink/openthread,JakubBrachTieto/openthread,LeZhang2016/openthread,turon/openthread,openthread/openthread,JakubBrachTieto/openthread,pvanhorn/openthread,fbsder/openthread,pvanhorn/openthread,JakubBrachTieto/openthread,JakubBrachTieto/openthread,openthread/openthread,lanyuwen/openthread,aeliot/openthread,aeliot/openthread,librasungirl/openthread,chshu/openthread,mszczodrak/openthread,srickardti/openthread,chshu/openthread,bukepo/openthread,openthread/openthread,aeliot/openthread,pvanhorn/openthread,bukepo/openthread,chshu/openthread,vaas-krish/openthread,abtink/openthread,erja-gp/openthread,chshu/openthread,fbsder/openthread,JakubBrachTieto/openthread,georgecpr/openthread,gandreello/openthread,fbsder/openthread,jwhui/openthread,xiaom-GitHub/openthread,srickardti/openthread,vaas-krish/openthread,erja-gp/openthread,srickardti/openthread,librasungirl/openthread,abtink/openthread,vaas-krish/openthread,vaas-krish/openthread,gandreello/openthread,xiaom-GitHub/openthread,turon/openthread,fbsder/openthread,lanyuwen/openthread,xiaom-GitHub/openthread,mszczodrak/openthread,georgecpr/openthread,gandreello/openthread,turon/openthread,lanyuwen/openthread,erja-gp/openthread,lanyuwen/openthread,pvanhorn/openthread