tools: disable libvchan build on NetBSD

tools: disable libvchan build on NetBSD

NetBSD doesn't have a gntdev, so libvchan is unable to build due to
the lack of the header files gntalloc.h.

This is the error:

gcc  -O1 -fno-omit-frame-pointer -m64 -g -fno-strict-aliasing
 -std=gnu99 -Wall -Wstrict-prototypes -Wdeclaration-after-statement
 -D__XEN_TOOLS__ -MMD -MF .init.o.d -fno-optimize-sibling-calls
 -I../include -I.
 -I/root/xen/xen-netbsd/tools/libvchan/../../tools/xenstore
 -I/root/xen/xen-netbsd/tools/libvchan/../../tools/include
 -I/root/xen/xen-netbsd/tools/libvchan/../../tools/libxc
 -I/root/xen/xen-netbsd/tools/libvchan/../../tools/include  -c -o init.o init.c
 init.c:45:30: fatal error: xen/sys/gntalloc.h: No such file or directory
compilation terminated.
gmake[3]: *** [init.opic] Error 1
gmake[3]: *** Waiting for unfinished jobs....
init.c:45:30: fatal error: xen/sys/gntalloc.h: No such file or directory
compilation terminated.

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Christoph Egger <Christoph.Egger@amd.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>