BUILD: cfgparse-ssl: Remove const from defpx param in keylog parsing function

defpx parameter is const in cfg parsing functions since the 2.4. It is not
const in 2.3 and below. This build issue was introduced by the commit
b32469435c ("BUILD: make tune.ssl.keylog available again").

This patch is 2.3-specific. Thus there is upstream commid ID. And no
backport is needed, except if the commit above is backported.
diff --git a/src/cfgparse-ssl.c b/src/cfgparse-ssl.c
index 24ef092..23949af 100644
--- a/src/cfgparse-ssl.c
+++ b/src/cfgparse-ssl.c
@@ -355,7 +355,7 @@
 }
 #else
 static int ssl_parse_global_keylog(char **args, int section_type, struct proxy *curpx,
-                                       const struct proxy *defpx, const char *file, int line,
+                                       struct proxy *defpx, const char *file, int line,
                                        char **err)
 {
 	memprintf(err, "'%s' requires at least OpenSSL 1.1.1.", args[0]);