include missing platform.hpp in curve_keygen

it was excluded, so HAVE_SODIUM would never be defined,
leading to curve_keygen always reporting "recompile with libsodium"

tools/curve_keygen.c

@@ -25,6 +25,7 @@
 */
 
 #include <assert.h>
+#include <platform.hpp>
 #include <zmq_utils.h>
 #ifdef HAVE_LIBSODIUM
 #   include <sodium.h>