add SecItemExport binding
diff --git a/src/cryptography/hazmat/bindings/commoncrypto/secimport.py b/src/cryptography/hazmat/bindings/commoncrypto/secimport.py
index 7ee535d..41a799f 100644
--- a/src/cryptography/hazmat/bindings/commoncrypto/secimport.py
+++ b/src/cryptography/hazmat/bindings/commoncrypto/secimport.py
@@ -75,6 +75,8 @@
                        const SecItemImportExportKeyParameters *,
                        SecKeychainRef, CFArrayRef *);
 OSStatus SecPKCS12Import(CFDataRef, CFDictionaryRef, CFArrayRef *);
+OSStatus SecItemExport(CFTypeRef, SecExternalFormat, SecItemImportExportFlags,
+                       const SecItemImportExportKeyParameters *, CFDataRef *);
 """
 
 MACROS = """