makefile.unix: make visibility=hidden the default
makefile.unix: make visibility=hidden the default

Change-Id: I22739f4db798bb95591839835b5368944396e887

webmproject/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,ttyangf/libwebp,webmproject/libwebp,ttyangf/libwebp,ttyangf/libwebp,ttyangf/libwebp,PKRoma/libwebp