RFR: 8323700: Add fontconfig requirement to building.md for Alpine Linux

[Jan Kratochvil]

Similar to JDK-8309287.

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Commit messages:
 - 8323700: Add fontconfig requirement to building.md for Alpine Linux

Changes: https://git.openjdk.org/jdk/pull/17415/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17415&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323700
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17415.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17415/head:pull/17415

PR: https://git.openjdk.org/jdk/pull/17415