pyzfs: Update ax_python_devel.m4 to serial 37

Fixes an obvious typo, where a variable was missing the required
leading dollar sign ($)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Martin Rüegg <martin.rueegg@metaworx.ch>
Closes #17480