add missing includes

[nim65s]

Hi,

On a standard ubuntu 20.04, with gcc 9.4 and eigen 3.3.7, eigen has a std::cerr somewhere and compilation raise some occurences of :

In file included from /usr/include/eigen3/unsupported/Eigen/IterativeSolvers:29,
                 from ../include/proxsuite/proxqp/sparse/views.hpp:21,
                 from ../include/proxsuite/proxqp/sparse/solver.hpp:20,
                 from ../include/proxsuite/proxqp/sparse/wrapper.hpp:12,
                 from ../test/src/sparse_qp.cpp:4:
/usr/include/eigen3/unsupported/Eigen/src/IterativeSolvers/ConstrainedConjGrad.h: In function ‘void Eigen::internal::constrained_cg(const TMatrix&, const CMatrix&, VectorX&, const VectorB&, const VectorF&, Eigen::IterationController&)’:
/usr/include/eigen3/unsupported/Eigen/src/IterativeSolvers/ConstrainedConjGrad.h:162:51: error: ‘cerr’ is not a member of ‘std’
  162 |     if (iter.noiseLevel() > 0 && transition) std::cerr << "CCG: transition\n";
      |                                                   ^~~~
/usr/include/eigen3/unsupported/Eigen/src/IterativeSolvers/ConstrainedConjGrad.h:30:1: note: ‘std::cerr’ is defined in header ‘<iostream>’; did you forget to ‘#include <iostream>’?
   29 | #include "../../../../Eigen/src/Core/util/NonMPL2.h"
  +++ |+#include <iostream>
   30 |

Here is a quick fix, but this is not super satisfying…

PS: I pushed to simple-robotics/devel by mistake, and reverted back a few seconds after that, sorry for the mess :(

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