LibPDF: Convert to east-const to comply with the recent style changes

Author: https://github.com/mattco98
Commit: 612b183703
Pull-request: https://github.com/SerenityOS/serenity/pull/7675
Reviewed-by: https://github.com/FireFox317
Reviewed-by: https://github.com/alimpfard

Userland/Libraries/LibPDF/Value.cpp

@@ -15,7 +15,7 @@ Value::~Value()
         m_as_object->unref();
 }
 
-Value& Value::operator=(const Value& other)
+Value& Value::operator=(Value const& other)
 {
     m_type = other.m_type;
     switch (m_type) {