Fix FTBFS with GCC-15

GCC 15 defaults to C23, which removed the unprototyped functions, and it causes freeglut to fail to build. Apply the upstream patch fixing issue.

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While including the patch, I noticed that adding it as the first of the series would cause the build to fail with:

dpkg-source: info: using patch list from debian/patches/series
dpkg-source: info: applying 10-conform-to-c23.patch
dpkg-source: info: applying 20_update_soversion.cmake
patching file CMakeLists.txt
Reversed (or previously applied) patch detected!  Skipping patch.
1 out of 1 hunk ignored
dpkg-source: info: the patch has fuzz which is not allowed, or is malformed
dpkg-source: info: if patch '20_update_soversion.cmake' is correctly applied by quilt, use 'quilt refresh' to update it
dpkg-source: info: if the file is present in the unpacked source, make sure it is also present in the orig tarball

The same would happen if I dropped all patches the existing series as well, because the CMakeLists.txt file wouldn't match the one from the upstream tarball:

dpkg-source: info: using source format '3.0 (quilt)'
dpkg-source: info: building freeglut using existing ./freeglut_3.4.0.orig.tar.gz
dpkg-source: info: local changes detected, the modified files are:
 freeglut/CMakeLists.txt
dpkg-source: hint: make sure the version in debian/changelog matches the unpacked source tree
dpkg-source: hint: you can integrate the local changes with dpkg-source --commit
dpkg-source: error: aborting due to unexpected upstream changes, see /tmp/freeglut_3.4.0-4.diff.3UcfJU

By git-log(1)ing the file, I noticed that two commits mistakenly updated both the patch file and the file itself. I reverted those changes so that they match once again to what upstream provides while still being correctly patched up for generating the binary package.

That made both issues go away, and I could include the new patch in whatever place in the series file.