Use a consistent preprocessor check for ENABLE_PORT_MAPPING_ISOLATOR.
Use a consistent preprocessor check for ENABLE_PORT_MAPPING_ISOLATOR.

There's no need to also check for Windows when testing the
ENABLE_PORT_MAPPING_ISOLATOR feature macro, because
ENABLE_PORT_MAPPING_ISOLATOR requires libnl3, which is a
Linux-specific features.

Review: https://reviews.apache.org/r/60901/

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