cpp: Expose gpgme_decrypt_result_t.is_mime through cpp API

* lang/cpp/src/descriptionresult.cpp (DescriptionResult::isMime): New.
* lang/cpp/src/descriptionresult.h: Update accordingly.

--
This exposes the is_mime metadata from a decryption result to users
of the C++ library.

GnuPG-bug-id: 6199
Signed-off-by: Carl Schwan <carl.schwan@gnupg.com>

lang/cpp/src/decryptionresult.cpp

@@ -122,6 +122,11 @@ bool GpgME::DecryptionResult::isDeVs() const
     return d && d->res.is_de_vs;
 }
 
+bool GpgME::DecryptionResult::isMime() const
+{
+    return d && d->res.is_mime;
+}
+
 const char *GpgME::DecryptionResult::fileName() const
 {
     return d ? d->res.file_name : nullptr ;

lang/cpp/src/decryptionresult.h

@@ -75,6 +75,7 @@ public:
     }
     bool isWrongKeyUsage() const;
     bool isDeVs() const;
+    bool isMime() const;
 
     const char *fileName() const;