clang: Fix warning reg. expansion to 'defined'

Clang produces the following warning. The warning is detailed here:
https://reviews.llvm.org/D15866. Fix the warning.

/home/pranith/devops/code/qemu/hw/display/qxl.c:507:5: warning: macro expansion producing 'defined' has undefined behavior [-Wexpansion-to-defined]
    ^
/home/pranith/devops/code/qemu/include/ui/qemu-spice.h:46:5: note: expanded from macro 'SPICE_NEEDS_SET_MM_TIME'
  (!defined(SPICE_SERVER_VERSION) || (SPICE_SERVER_VERSION < 0xc06))
    ^
/home/pranith/devops/code/qemu/hw/display/qxl.c:1074:5: warning: macro expansion producing 'defined' has undefined behavior [-Wexpansion-to-defined]
    ^
/home/pranith/devops/code/qemu/include/ui/qemu-spice.h:46:5: note: expanded from macro 'SPICE_NEEDS_SET_MM_TIME'
  (!defined(SPICE_SERVER_VERSION) || (SPICE_SERVER_VERSION < 0xc06))

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
