RFR: 8309287: Add fontconfig requirement to building.md for Debian

[Jan Kratochvil]

After following all the commands in `building.md` I still get on Debian 12 x86_64:


checking for fontconfig/fontconfig.h... no
configure: error: Could not find fontconfig!

-------------

Commit messages:
 - 8309287: Add fontconfig requirement to building.md for Debian

Changes: https://git.openjdk.org/jdk/pull/14261/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14261&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309287
  Stats: 4 lines in 2 files changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14261.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14261/head:pull/14261

PR: https://git.openjdk.org/jdk/pull/14261