patch 8.1.0521: cannot build with +eval but without +quickfix

Problem:    Cannot build with +eval but without +quickfix.
Solution:   Remove #ifdef for e_stringreq. (John Marriott)

src/evalfunc.c

@@ -29,9 +29,7 @@
 #endif
 
 static char *e_listarg = N_("E686: Argument of %s must be a List");
-#ifdef FEAT_QUICKFIX
 static char *e_stringreq = N_("E928: String required");
-#endif
 
 #ifdef FEAT_FLOAT
 static void f_abs(typval_T *argvars, typval_T *rettv);

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    521,
 /**/
     520,
 /**/