Wrap bootcamp DEBUG statement with curly braces

This fixes a -Wempty-body warning with gcc 6.3.0 when PART_DEBUG is undefined.

MFC after: 3 days
Reported by: Jenkins (FreeBSD-head-amd64-gcc job)
Tested with: amd64-gcc-6.3.0 (devel/amd64-xtoolchain-gcc)
Sponsored by: Dell EMC Isilon