Add mbedtls_ecdh_can_do

All curves can currently do ECDH, but to make the API symmetric and
future-proof, add mbedtls_ecdh_can_do() to go with mbedtls_ecdsa_can_do().
diff --git a/library/ecdh.c b/library/ecdh.c
index 66a2d16..648becb 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -63,6 +63,13 @@
 #endif
 }
 
+int mbedtls_ecdh_can_do( mbedtls_ecp_group_id gid )
+{
+    /* At this time, all groups support ECDH. */
+    (void) gid;
+    return 1;
+}
+
 #if !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
 /*
  * Generate public key (restartable version)