Adjust where UNAME_r is defined for ports builds
In followup to 80ab50e1de19ca125f05a13937c796d48c4edd4a,
export UNAME_r in Makefile.inc1 instead of Makefile.vm.
MFC after: 3 days
MFC with: 80ab50e1de19
Sponsored by: Rubicon Communications, LLC ("Netgate")