[foreign-memaccess+abi] RFR: Fix VarHandleTestExact

[Maurizio Cimadamore]

This small patch fixes a compilation error in a j.l.i test using memory access var handles.

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Commit messages:
 - Fix VarHandleTestExact

Changes: https://git.openjdk.java.net/panama-foreign/pull/582/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=582&range=00
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/582.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/582/head:pull/582

PR: https://git.openjdk.java.net/panama-foreign/pull/582