BUILD: ssl: fix a warning when built with openssl < 1.0.2

src/ssl_sock.c:2928:12: warning: ‘ssl_sock_is_ckch_valid’ defined but not used [-Wunused-function]
 static int ssl_sock_is_ckch_valid(struct cert_key_and_chain *ckch)

This function is only used with openssl >= 1.0.2, this patch adds a
condition to build the function.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 74c6342..726ffb2 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -2925,11 +2925,12 @@
 
 /* checks if a key and cert exists in the ckch
  */
+#if HA_OPENSSL_VERSION_NUMBER >= 0x1000200fL
 static int ssl_sock_is_ckch_valid(struct cert_key_and_chain *ckch)
 {
 	return (ckch->cert != NULL && ckch->key != NULL);
 }
-
+#endif
 
 /* Loads the contents of a crt file (path) into a cert_key_and_chain
  * This allows us to carry the contents of the file without having to