use-retval for localtime() (#3935)

* use-retval for localtime()

* Adjust test to avoid new warning

cfg/std.cfg

@@ -3159,6 +3159,7 @@ The obsolete function 'gets' is called. With 'gets' you'll get a buffer overrun
   </function>
   <!-- struct tm * localtime(const time_t *tp); -->
   <function name="localtime,std::localtime">
+    <use-retval/>
     <returnValue type="struct tm *"/>
     <noreturn>false</noreturn>
     <leak-ignore/>

test/testfunctions.cpp

@@ -374,7 +374,7 @@ private:
         check("void f()\n"
               "{\n"
               "    time_t t = 0;"
-              "    std::localtime(&t);\n"
+              "    auto lt = std::localtime(&t);\n"
               "}");
         ASSERT_EQUALS("[test.cpp:3]: (portability) Non reentrant function 'localtime' called. For threadsafe applications it is recommended to use the reentrant replacement function 'localtime_r'.\n", errout.str());