cpp: Fix Error::hasSystemError

* lang/cpp/src/context.cpp (Error::hasSystemError): Invert logic to
do what it says.

--
This is safe because according to codesearch it is only used
in QGpgME

lang/cpp/src/context.cpp

@@ -155,7 +155,7 @@ int Error::toErrno() const
 // static
 bool Error::hasSystemError()
 {
-    return gpgme_err_code_from_syserror() == GPG_ERR_MISSING_ERRNO ;
+    return gpgme_err_code_from_syserror() != GPG_ERR_MISSING_ERRNO ;
 }
 
 // static