if_cgem: Make ext_resources non-optional

EXT_RESOURCES have been introduced in 12-CURRENT and all supported
releases have it enabled in their kernel config.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33825

(cherry picked from commit d46fd9e855a2212f59ae35e84162d2d1a44f2035)