NEED_VARARGS bit for end.c and files.c

avoid build failure in the event that NEED_VARARGS is already defined
on the compiler command line.

src/end.c

@@ -3,7 +3,9 @@
 /*-Copyright (c) Robert Patrick Rankin, 2012. */
 /* NetHack may be freely redistributed.  See license for details. */
 
+#ifndef NEED_VARARGS
 #define NEED_VARARGS /* comment line for pre-compiled headers */
+#endif
 
 #include "hack.h"
 #ifndef NO_SIGNAL

src/files.c

@@ -3,7 +3,9 @@
 /*-Copyright (c) Derek S. Ray, 2015. */
 /* NetHack may be freely redistributed.  See license for details. */
 
+#ifndef NEED_VARARGS
 #define NEED_VARARGS
+#endif
 
 #include "hack.h"
 #include "dlb.h"