Pixman used to be included directly in QEMU_CFLAGS and therefore the
include path was added to every compiler invocation.  Now that (just
like basically everything else) it is a separate dependency, we
need to add it to all build target, especially UI modules that need
it due to their including ui/console.h.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>