randr: use randr: prefix in ErrorF()

To provide some information about the origin of the message.

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

randr/rrcrtc.c

@@ -428,7 +428,7 @@ rrCreateSharedPixmap(RRCrtcPtr crtc, int width, int height,
 
     ret = pScrPriv->rrCrtcSetScanoutPixmap(crtc, spix);
     if (ret == FALSE) {
-        ErrorF("failed to set shadow slave pixmap\n");
+        ErrorF("randr: failed to set shadow slave pixmap\n");
         return FALSE;
     }