event_pipe, test: explicitly ignore write() return value

Some compilers are very picky, but we really aren't interested in the
return value.

test/software_volume.c

@@ -65,6 +65,6 @@ int main(int argc, char **argv)
 			return 2;
 		}
 
-		write(1, buffer, nbytes);
+		G_GNUC_UNUSED ssize_t ignored = write(1, buffer, nbytes);
 	}
 }