net/freerdp: fix build on FreeBSD 11.x

Apparently this hadn't been caught in my test matrix -- it seems that later
versions of FreeBSD have a getmntent() definition that masked this error.

mntent_compat.c has been adopted from devel/fam, though a better solution
should be sought out going into the future.

Approved by: koobs (mentor, implicit, just-fix-it)
MFH: 2020Q2 (blanket: build fix)