makefile.unix: normalize image decode lib name
makefile.unix: normalize image decode lib name

libexample_dec -> libimagedec

+ add it to OUT_LIBS so it can be removed by the clean target

Change-Id: I3ef4711e261ead2e5d437182ad3b495ae3da7cbc

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