xdrgen: Exit status should be zero on success

To use xdrgen in Makefiles, it needs to exit with a zero status if
the compilation worked. Otherwise the make command fails with an
error.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

tools/net/sunrpc/xdrgen/xdrgen

@@ -128,5 +128,7 @@ There is NO WARRANTY, to the extent permitted by law.""",
 try:
     if __name__ == "__main__":
         sys.exit(main())
-except (SystemExit, KeyboardInterrupt, BrokenPipeError):
+except SystemExit:
+    sys.exit(0)
+except (KeyboardInterrupt, BrokenPipeError):
     sys.exit(1)