net/freerdp3: Fix a undeclared variable issue with NEON builds

https://pkg-status.freebsd.org/ampere3/data/132arm64-default/74acf3311b89/logs/freerdp3-3.3.0.log
https://pkg-status.freebsd.org/ampere3/data/140arm64-default/74acf3311b89/logs/freerdp3-3.3.0.log

/wrkdirs/usr/ports/net/freerdp3/work/freerdp-3.3.0/libfreerdp/primitives/prim_YUV_neon.c:209:10: error: use of undeclared identifier 'x'

for (; x < nWidth; x++)
       ^

/wrkdirs/usr/ports/net/freerdp3/work/freerdp-3.3.0/libfreerdp/primitives/prim_YUV_neon.c:209:22: error: use of undeclared identifier 'x'

for (; x < nWidth; x++)
                   ^

/wrkdirs/usr/ports/net/freerdp3/work/freerdp-3.3.0/libfreerdp/primitives/prim_YUV_neon.c:238:8: error: use of undeclared identifier 'x'

if (x % 2)
    ^

... cut ...

fatal error: too many errors emitted, stopping now [-ferror-limit=]
9 warnings and 20 errors generated.

Reported by: pkg-fallout
Approved by: arrowd (mentor, implicit)