platform: fix build

  ./src/libnm-platform/nm-platform.h:1315: undefined reference to `_nm_utils_ip4_prefix_to_netmask'
  make[2]: *** [Makefile:11063: src/core/platform/tests/test-cleanup-fake] Error 1
  ./src/libnm-platform/nm-platform.h:1315: undefined reference to `_nm_utils_ip4_prefix_to_netmask'
  make[2]: *** [Makefile:11070: src/core/platform/tests/test-cleanup-linux] Error 1
57 jobs for lr/el9-build in 36 minutes and 58 seconds (queued for 4 minutes and 45 seconds)
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