gdb/testsuite: fix undefined variable in gdb.ada/scalar_storage.exp

Commit:

  commit be382ece165eefa3e65f61bfb6b2aa2ee95dd6b4
  Date:   Wed Feb 12 09:35:26 2025 -0700

      Check for compiler support in scalar_storage.exp

Introduced an undefined variable use in gdb.ada/scalar_storage.exp,
fixed by this commit.
1 file changed