graf2d/x11, graf3d/x3d: Remove K&R function definitions

[hahnjo]

Soon-to-be-released Clang 15 complains:

a function definition without a prototype is deprecated in all versions of C and is not supported in C2x [-Wdeprecated-non-prototype]

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds

[phsft-bot]

Build failed on ROOT-performance-centos8-multicore/cxx17. Running on olbdw-01.cern.ch:/data/sftnight/workspace/root-pullrequests-build See console output.

Failing tests:

• projectroot.runtutorials.tutorial_spectrum_Background_order

[phsft-bot]

Build failed on mac11/cxx14. Running on macphsft23.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build See console output.

Failing tests:

• projectroot.tree.dataframe.test.gtest_tree_dataframe_test_dataframe_snapshot

[phsft-bot]

Build failed on mac1015/cxx17. Running on macitois21.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build See console output.

Failing tests:

• projectroot.roottest.python.distrdf.dask.roottest_python_distrdf_dask_test_all
• projectroot.roottest.python.distrdf.spark.roottest_python_distrdf_spark_test_all