Adjust dependencies for EC extensions

The Thread spec says we need those for EC J-PAKE too.
However, we won't be using the information, so we can skip the parsing
functions in an EC J-PAKE only config; keep the writing functions in order to
comply with the spec.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index ca1e7b8..afac3e1 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -1976,7 +1976,8 @@
 }
 #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
-#if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C)
+#if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
+    defined(MBEDTLS_ECJPAKE_C)
 static void ssl_write_supported_point_formats_ext( mbedtls_ssl_context *ssl,
                                                    unsigned char *buf,
                                                    size_t *olen )
@@ -2004,7 +2005,7 @@
 
     *olen = 6;
 }
-#endif /* MBEDTLS_ECDH_C || MBEDTLS_ECDSA_C */
+#endif /* MBEDTLS_ECDH_C || MBEDTLS_ECDSA_C || MBEDTLS_ECJPAKE_C */
 
 #if defined(MBEDTLS_SSL_ALPN )
 static void ssl_write_alpn_ext( mbedtls_ssl_context *ssl,
@@ -2290,7 +2291,8 @@
     ext_len += olen;
 #endif
 
-#if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C)
+#if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
+    defined(MBEDTLS_ECJPAKE_C)
     ssl_write_supported_point_formats_ext( ssl, p + 2 + ext_len, &olen );
     ext_len += olen;
 #endif