net/freerdp3: fix "sysinfo" breakage

After upstream commit
https://github.com/FreeRDP/FreeRDP/commit/4732f379d4d723378a5ec4948b00061cebbd67af
net/freerdp3 build fails in enveronments without devel/libsysinfo installed.
But with devel/libsysinfo installed net/freerdp3 grab silently
LOCALBASE/include/sys/sysinfo.h, build without errors, but sdl-freerdp
and wlfreerdp3 run failed with error:
ld-elf.so.1: /usr/local/lib/libwinpr3.so.3: Undefined symbol "sysinfo"

Upstream check supported version of the posix using _POSIX_C_SOURCE define.
This doesn't work on FreeBSD.
Together with upstream trying to find correct solution...

Approved by: arrowd (mentor)