Added libidn to libpurple dependencies

Without this line autogen.s complains:

==> ./autogen.sh --disable-dependency-tracking --disable-gtkui --disable-consoleui --disable-perl --disable-static --enable-shared --enable-cyrus-sasl --with-static-prpls=gg,irc,jab
no
configure: error: 
GNU Libidn development headers not found.
Use --disable-idn if you do not need it.