The inclusion of bn.h from the engine.h API header has been deprecated, so
the engine implementations need to include bn.h to manipulate bignums.
diff --git a/engines/e_cswift.c b/engines/e_cswift.c
index b73126b..0f9c1b2 100644
--- a/engines/e_cswift.c
+++ b/engines/e_cswift.c
@@ -66,6 +66,7 @@
 #include <openssl/dsa.h>
 #include <openssl/dh.h>
 #include <openssl/rand.h>
+#include <openssl/bn.h>
 
 #ifndef OPENSSL_NO_HW
 #ifndef OPENSSL_NO_HW_CSWIFT