Remove xml warning

[mikaelarguedas]

This PR uses xml version 2 for cppcheck output.

Rationale:
cppcheck deprecated usage of version 1 in the last releases, and will completely drop support in v1.81.
We started to see deprecation warnings show on Mac and Windows CIs (cppcheck: XML format version 1 is deprecated and will be removed in cppcheck 1.81. Use '--xml-version=2') e.g. http://ci.ros2.org/view/nightly/job/nightly_osx_debug/373

Tested with a few errors for either same or different kind and the generated xunit file is identical as with version 1.
Errors tested:

void f(char *p) {
    if (p+1){}
}

bool using_introspection_c_typesupport(const char * typesupport_identifier)
{
  double * a = reinterpret_cast<double *>(malloc(10 * sizeof(double)));
  (void)a;
  return true;
}

[dirk-thomas]

Since errors is only used once this could stay in one line as before: for error in root.find('errors'):

[mikaelarguedas]

fixed in c279be1