x11-toolkits/v: Fix build with Clang 6

vchkboxc.cxx:43:4: error: constant expression evaluates to 255 which cannot be narrowed to type 'char' [-Wc++11-narrowing]

0xff, 0x0f, 0x03, 0x0c, 0x05, 0x0a, 0x09, 0x09, 0x91, 0x08, 0x61, 0x08,
^~~~

vchkboxc.cxx:43:4: note: insert an explicit cast to silence this issue

0xff, 0x0f, 0x03, 0x0c, 0x05, 0x0a, 0x09, 0x09, 0x91, 0x08, 0x61, 0x08,
^~~~
static_cast<char>( )

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/v-1.90_6.log