Quiet warning with -Wformat-signedness

Is actually a redundant cast since Blender uses -funsigned-char, however I think it's fine to be explicit about it in new code so cast is required to make compiler happy. Am not a fan of -funsigned-char anyway...

source/blender/editors/interface/interface_widgets.c

@@ -2342,7 +2342,7 @@ static void ui_draw_but_HSVCIRCLE(uiBut *but, uiWidgetColors *wcol, const rcti *
 	glEnable(GL_BLEND);
 	glEnable(GL_LINE_SMOOTH);
 
-	immUniformColor3ubv(wcol->outline);
+	immUniformColor3ubv((unsigned char *)wcol->outline);
 	imm_draw_lined_circle(pos, centx, centy, radius, tot);
 
 	glDisable(GL_BLEND);