qemumonitortestutils: Fix G_GNUC_PRINTF annotation of qemuMonitorTestAddErrorResponse()

The qemuMonitorTestAddErrorResponse() function is a printf-like
function. But the annotation was mistakenly done in .c file
instead of corresponding .h file rendering the annotation
ineffective. Move the annotation to the header file.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>