xf86-input-vmmouse: Make xf86-input-vmmouse available only to x86/x86_64 architectures

The xf86-input-vmmouse source code fails to compile intentionally on any
architecture but x86/x86_64. The following error is printed when compiling for a
different architecture:

vmmouse_proto.h:62:2: error: #error The vmmouse protocol is only supported on
x86 architectures.
vmmouse_proto.c:117:2: error: #error "VMMouse is only supported on x86 and
x86-64.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>