[foreign-jextract] RFR: 8275875: jextract branch fails to build due to javac serialization warning

[Athijegannathan Sundararajan]

Changed Path type to String type for the instance field.

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Commit messages:
 - 8275875: jextract branch fails to build due to javac serialization warning

Changes: https://git.openjdk.java.net/panama-foreign/pull/604/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=604&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275875
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/604.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/604/head:pull/604

PR: https://git.openjdk.java.net/panama-foreign/pull/604