autotools: Fix doc_DATA install.

autotools: Fix doc_DATA install.

Both the generated format specification document and the static
html documents depend on the logo in fish_xiph_org.png.

Unfortunately we can't list them both in their respective
dependencies because both are passed to doc_DATA for installation
and automake doesn't seem to remove duplicates, causing
/usr/bin/install to complain about the overwrite.

I couldn't find a good solution to this. Wrapping the doc_DATA
value in GNU make's $(sort /list/) function would work, but
automake warns about the non-portable construct. Instead, I've
just removed it from the spec dependency list and appended it
manually.