Fix unittest compilation with --disable-curve25519

This is a fix for 9700, which we already fixed in 0.2.5.x, but which
got left in 0.2.4.x.

This is a partial backport of a0a855d586

changes/bug9700

@@ -0,0 +1,3 @@
+  o Minor bugfixes (compilation):
+    - Fix a compilation error when compiling with --disable-cuve25519.
+      Fixes bug 9700; bugfix on 0.2.4.17-rc.

src/test/test.c

@@ -56,12 +56,12 @@ double fabs(double x);
 #include "memarea.h"
 #include "onion.h"
 #include "onion_tap.h"
+#include "onion_ntor.h"
 #include "policies.h"
 #include "rephist.h"
 #include "routerparse.h"
 #ifdef CURVE25519_ENABLED
 #include "crypto_curve25519.h"
-#include "onion_ntor.h"
 #endif
 
 #ifdef USE_DMALLOC