pgsql: test_custom_stats: Fix compilation warning

test_custom_stats: Fix compilation warning

I have fat-fingered an error message related to an offset while
switching the code to use pgoff_t.  Let's switch to the same error
message used in the rest of the tree for similar failures with fseeko(),
instead.

Per buildfarm members running macos: longfin, sifaka and indri.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/33980eaa6df0b1358ba2e70edbe9efd56dbd7fa6

Modified Files
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src/test/modules/test_custom_stats/test_custom_var_stats.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)