Move -lm to the end of the line, so that it can be picked up as a
dependency by pixman in the static build case.

Reviewed-by: Peter Maydell <peter.maydell@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>