[CMake] Require eigen 3.3

Otherwise:

…/unittest/eiquadprog-rt.cpp:180:24: error:
call of overloaded 'Matrix(int)' is ambiguous
10 jobs for devel in 13 minutes and 36 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed doc-coverage #41092

00:04:16

passed format #41018

00:00:21

passed robotpkg-eiquadprog-16.04-debug #41012

00:06:16

passed robotpkg-eiquadprog-16.04-release #41011

00:05:25

passed robotpkg-eiquadprog-18.04-debug #41091

00:03:07

passed robotpkg-eiquadprog-18.04-release #41013

00:05:40

passed robotpkg-eiquadprog-stretch-debug #41016

00:05:36

passed robotpkg-eiquadprog-stretch-release #41015

00:09:19

failed doc-coverage #41017

00:00:10

failed robotpkg-eiquadprog-18.04-debug #41014
allowed to fail

00:00:10