find: port to Ubuntu 20.04 cross-compiling for OpenWRT 21.02

In the above build environment, HAVE_ENDPWENT seems to be defined to 0
instead of being #undef'ed.  Hence the build fails:

  parser.c: In function 'parse_user':
  parser.c:75:20: error: expected expression before ')' token
     75 | # define endpwent ()
        |                    ^
  parser.c:2463:7: note: in expansion of macro 'endpwent'
   2463 |       endpwent ();
        |       ^~~~~~~~

* find/parser.c (HAVE_ENDGRENT,HAVE_ENDPWENT): Change from #ifndef to
"#if !", and define the replacement code to "(void) 0".

Fixes https://github.com/openwrt/packages/issues/17912

find/parser.c

@@ -67,12 +67,12 @@
 #include "findutils-version.h"
 #include "system.h"
 
-
-#ifndef HAVE_ENDGRENT
-# define endgrent ()
+#if ! HAVE_ENDGRENT
+# define endgrent() ((void) 0)
 #endif
-#ifndef HAVE_ENDPWENT
-# define endpwent ()
+
+#if ! HAVE_ENDPWENT
+# define endpwent() ((void) 0)
 #endif
 
 static bool parse_accesscheck   (const struct parser_table*, char *argv[], int *arg_ptr);