makefile.unix: clean up after extras target
makefile.unix: clean up after extras target

Change-Id: I3e2d259473db9f3649d18120513f8edcba64c5e6

rxl194/libwebp,ttyangf/libwebp,webmproject/libwebp,PKRoma/libwebp,yohunl/libwebp,rxl194/libwebp,redengineer/libwebp,zofuthan/libwebp,webmproject/libwebp,ya7lelkom/libwebp,ttyangf/libwebp,zofuthan/libwebp,webmproject/libwebp,PKRoma/libwebp,Aexyn/libwebp,imazen/libwebp,Aexyn/libwebp,PKRoma/libwebp,Aexyn/libwebp,redengineer/libwebp,zofuthan/libwebp,rxl194/libwebp,imazen/libwebp,imazen/libwebp,yohunl/libwebp,webmproject/libwebp,yohunl/libwebp,yohunl/libwebp,ttyangf/libwebp,ttyangf/libwebp,ya7lelkom/libwebp,webmproject/libwebp,ttyangf/libwebp,rxl194/libwebp,PKRoma/libwebp,imazen/libwebp,Aexyn/libwebp,imazen/libwebp,redengineer/libwebp,zofuthan/libwebp,redengineer/libwebp,PKRoma/libwebp,redengineer/libwebp,ya7lelkom/libwebp,zofuthan/libwebp,Aexyn/libwebp,rxl194/libwebp,PKRoma/libwebp,ya7lelkom/libwebp,yohunl/libwebp,ya7lelkom/libwebp