commit a93a3af9 introduces use of PIXMAN_TYPE_RGBA, but it's only available
in pixman >= 0.21.8. If pixman doesn't meet the version requirement, qemu
will fail to build with following message:

qemu/ui/qemu-pixman.c: In function ‘qemu_pixelformat_from_pixman’:
qemu/ui/qemu-pixman.c:42: error: ‘PIXMAN_TYPE_RGBA’ undeclared (first use in this function)
qemu/ui/qemu-pixman.c:42: error: (Each undeclared identifier is reported only once
qemu/ui/qemu-pixman.c:42: error: for each function it appears in.)

This patch fixes the problem by checking the pixman version.
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>