Revert "libmodbus: revert to 2.0.4 to unbreak collectd support"

Revert "libmodbus: revert to 2.0.4 to unbreak collectd support"

This reverts commit 98d5c3a15acb46ffa557afbda30f2760ad66168c.

If collectd wants to stay on their own very outdated v4.10.x, that's
fine, but they should not drag other packages down around with them.
Support for modern versions of libmodbus has existed in collectd since
v5.0.0, released 2011-03-28, so the actively maintained version of
libmodbus in OpenWrt's modern package feed should remain at the modern
versions in use by third party applications.

This restores libmodbus to the existing version used
throughout AA and BB prior to this collectd "fix"

Signed-off-by: Karl Palsson <karlp@remake.is>