Fix build when use old GnuTLS release (< 3.1)

FossilOrigin-Name: a707b54e17b5c62455abfdc7857ee9b4196f963848ff5401d9099ac8aeaa9eb9

src/capwap/dtls_gnutls.c

@@ -117,7 +117,11 @@ struct dtls_gnutls_data *dtls_gnutls_data_create(struct conn *conn,int config)
 
 
 	int bits;
+#if GNUTLS_VERSION_NUMBER >= 0x030100
 	bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH, GNUTLS_SEC_PARAM_INSECURE);
+#elseif
+	bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH, GNUTLS_SEC_PARAM_WEAK);
+#endif
         /* Generate Diffie-Hellman parameters - for use with DHE
          * kx algorithms. When short bit length is used, it might
          * be wise to regenerate parameters often.