wireshark: Use ${exec_prefix} instead of ${prefix}

wireshark: Use ${exec_prefix} instead of ${prefix}

${exec_prefix} and ${prefix} point to the same directory in
most setups, but when that's not the case the former should
be used for architecture-dependent data such as shared objects,
which makes it the best fit for our Wireshark dissector.

While at it, change all uses of $(var) to ${var}: they are
absolutely identicaly as far as make's concerned, but autoconf
itself seems to prefer the latter form so we might as well
follow suit.