RFR: 8272332: --with-harfbuzz=system doesn't add -lharfbuzz after JDK-8255790

[Severin Gehwolf]

Please review this simple build fix to correct the type done in JDK-8255790. After the patch correct external library is added to the `libfontmanager.so` link command when building with `--with-harfbuzz=system`.

Thoughts?

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Commit messages:
 - 8272332: --with-harfbuzz=system doesn't add -lharfbuzz after JDK-8255790

Changes: https://git.openjdk.java.net/jdk/pull/5091/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5091&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272332
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5091.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5091/head:pull/5091

PR: https://git.openjdk.java.net/jdk/pull/5091