Add build-depend on pkg-config to find urdfdom

[scpeters]

I noticed that the 10.7.0~pre1 debbuilds failed to compile some tests with complaints about finding urdf headers:

• https://build.osrfoundation.org/job/sdformat10-debbuilder/242

src/parser_urdf.cc:30:10: fatal error: urdf_model/model.h: No such file or directory
 #include <urdf_model/model.h>
          ^~~~~~~~~~~~~~~~~~~~
compilation terminated.
src/CMakeFiles/UNIT_ParamPassing_TEST.dir/build.make:257: recipe for target
 'src/CMakeFiles/UNIT_ParamPassing_TEST.dir/parser_urdf.cc.o' failed

It turns out there was a bug in 10.7.0pre1 when USE_INTERNAL_URDF == TRUE, and the debbuilds were unintentionally failing to find the external urdfdom since pkg-config was not found. I've added pkg-config as a build-depend in the control file here, which should fix the 10.7.0pre1 builds.

Separately, I've submitted a fix for the compilation error when USE_INTERNAL_URDF == TRUE in gazebosim/sdformat#800

[scpeters]

testing

• https://build.osrfoundation.org/job/sdformat10-debbuilder/245/

[scpeters]

testing

• https://build.osrfoundation.org/job/sdformat10-debbuilder/245/

it's unstable because the sdformat10-all cmake files are not installed. that should be fixed by 910cfcc

testing again:

• https://build.osrfoundation.org/job/sdformat10-debbuilder/246/

[scpeters]

testing

• https://build.osrfoundation.org/job/sdformat10-debbuilder/245/

it's unstable because the sdformat10-all cmake files are not installed. that should be fixed by 910cfcc

testing again:

• https://build.osrfoundation.org/job/sdformat10-debbuilder/246/

oddly, the FindIgnURDFDOM module seems to work on 20.04 but not 18.04 I think it's not the find module, but the logic for which packages are installed as dependencies on the system seems to differ by platform, and I don't know why

[scpeters]

I've filed gazebo-tooling/release-tools#606 to track the issue of using the system liburdfdom in our debian binary packages in case we don't have time to resolve this for the final libsdformat10 release.

[scpeters]

we'll address this for other versions