Make build shut up about system() without return value check.

    st.c:1321:2: warning: ignoring return value of function declared with warn_unused_result attribute [-Wunused-result]
            system(cmd);
            ^~~~~~ ~~~

Debatable whether an error here should case exit(EXIT_FAILURE). Just
preserving the existing behaviour for now.

st.c

@@ -1276,7 +1276,8 @@ stty(void)
 		siz-= n + 1;
 	}
 	*q = '\0';
-	system(cmd);
+	if (system(cmd) != 0)
+	    perror("Couldn't call stty");
 }
 
 void