posix.cfg: improve configuration for dlsym()

cfg/posix.cfg

@@ -95,6 +95,7 @@
   <function name="dlsym">
     <noreturn>false</noreturn>
     <leak-ignore/>
+    <use-retval/>
     <returnValue type="void*"/>
     <arg nr="1">
       <not-uninit/>

test/cfg/posix.c

@@ -343,6 +343,8 @@ void dl(const char* libname, const char* func)
     // cppcheck-suppress uninitvar
     // cppcheck-suppress unreadVariable
     void* sym = dlsym(lib, funcname);
+    // cppcheck-suppress ignoredReturnValue
+    dlsym(lib, "foo");
     void* uninit;
     // cppcheck-suppress uninitvar
     dlclose(uninit);