multipart-parser: eliminate temporary to emplace_back

Fix the following clang-tidy warning:

```
../include/multipart_parser.hpp:108:50: error: unnecessary temporary object created while calling emplace_back [modernize-use-emplace,-warnings-as-errors]
  108 |                         mime_fields.emplace_back(FormPart{});
      |                                                  ^~~~~~~~~~
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I362b4ad7f90f80a7746b79d643e3a7c5ff1db78c
diff --git a/include/multipart_parser.hpp b/include/multipart_parser.hpp
index 94983f2..9adf564 100644
--- a/include/multipart_parser.hpp
+++ b/include/multipart_parser.hpp
@@ -105,7 +105,7 @@
                             return ParserError::ERROR_BOUNDARY_LF;
                         }
                         index = 0;
-                        mime_fields.emplace_back(FormPart{});
+                        mime_fields.emplace_back();
                         state = State::HEADER_FIELD_START;
                         break;
                     }
@@ -305,7 +305,7 @@
                 {
                     // unset the PART_BOUNDARY flag
                     flags = Boundary::NON_BOUNDARY;
-                    mime_fields.emplace_back(FormPart{});
+                    mime_fields.emplace_back();
                     state = State::HEADER_FIELD_START;
                     return ParserError::PARSER_SUCCESS;
                 }