BUG/MINOR: extcheck: proxy_parse_extcheck() must take a const for the defproxy

The default proxy was passed as a variable, which in addition to being
a PITA to deal with in the config parser, doesn't feel safe to use when
it ought to be const.

This will only affect new code so no backport is needed.
diff --git a/src/extcheck.c b/src/extcheck.c
index 6665e96..9ef87c5 100644
--- a/src/extcheck.c
+++ b/src/extcheck.c
@@ -648,7 +648,7 @@
 	return -1;
 }
 
-int proxy_parse_external_check_opt(char **args, int cur_arg, struct proxy *curpx, struct proxy *defpx,
+int proxy_parse_external_check_opt(char **args, int cur_arg, struct proxy *curpx, const struct proxy *defpx,
 				   const char *file, int line)
 {
 	int err_code = 0;