tests: Move ecdh stub out of autogenerated stubs

I ran into an issue when regenerating the stubs without
EXPERIMENTAL_FEATURES which then broke builds that have it set. Moving
the missing stub out.

common/test/run-sphinx.c

@@ -56,9 +56,6 @@ struct amount_sat amount_tx_fee(u32 fee_per_kw UNNEEDED, size_t weight UNNEEDED)
 /* Generated stub for bigsize_put */
 size_t bigsize_put(u8 buf[BIGSIZE_MAX_LEN] UNNEEDED, bigsize_t v UNNEEDED)
 { fprintf(stderr, "bigsize_put called!\n"); abort(); }
-/* Generated stub for ecdh */
-void ecdh(const struct pubkey *point UNNEEDED, struct secret *ss UNNEEDED)
-{ fprintf(stderr, "ecdh called!\n"); abort(); }
 /* Generated stub for fromwire_amount_msat */
 struct amount_msat fromwire_amount_msat(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_amount_msat called!\n"); abort(); }
@@ -95,6 +92,12 @@ void towire_tlv(u8 **pptr UNNEEDED,
 { fprintf(stderr, "towire_tlv called!\n"); abort(); }
 /* AUTOGENERATED MOCKS END */
 
+#if EXPERIMENTAL_FEATURES
+/* Generated stub for ecdh */
+void ecdh(const struct pubkey *point UNNEEDED, struct secret *ss UNNEEDED)
+{ fprintf(stderr, "ecdh called!\n"); abort(); }
+#endif
+
 extern secp256k1_context *secp256k1_ctx;
 
 static struct secret secret_from_hex(const char *hex)