Add missing include to Graph.hh

[chapulina]

🦟 Bug fix

Summary

This was causing SDFormat to compile locally for me, using gazebosim/sdformat#803

/home/chapulina/dev_focal/ws_garden/install/include/ignition/math7/ignition/math/graph/Graph.hh: In constructor _ignition::math::v7::graph::
Graph<V, E, EdgeType>::Graph(const std::vector<ignition::math::v7::graph::Vertex<V> >&, const std::vector<ignition::math::v7::graph::EdgeInitializer<E> >&)_:                                                                                                                           
/home/chapulina/dev_focal/ws_garden/install/include/ignition/math7/ignition/math/graph/Graph.hh:118:16: error: _cerr_ is not a member of _std_                                                                                                                                          
  118 |           std::cerr << "Invalid vertex with Id [" << v.Id() << "]. Ignoring."                                                       
      |                ^~~~                                                                                                                 
/home/chapulina/dev_focal/ws_garden/install/include/ignition/math7/ignition/math/graph/Graph.hh:29:1: note: _std::cerr_ is defined in header
 _<iostream>_; did you forget to _#include <iostream>_?                                                                                     
   28 | #include <ignition/math/config.hh>                                                                                                  
  +++ |+#include <iostream>                                                                                                                 
   29 | #include "ignition/math/graph/Edge.hh"  

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #342 (3290815) into main (132b0e4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #342   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files          67       67           
  Lines        6077     6077           
=======================================
  Hits         6033     6033           
  Misses         44       44           

Impacted Files	Coverage Δ
include/ignition/math/graph/Graph.hh	100.00% <ø> (ø)

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