Output xml decleration for Qt Linguist files (fixes #3198).

lupdate uses »"« in the xml decleration but the tested versions of lxml (2.1.0, 3.2.1) emitted »'«, so I specified the xml header manually.

I tested multiple versions of lupdate with the following source file (main.cpp):
```
#include <QDebug>
#include <QCoreApplication>

int main(int argc, char *argv[])
{
  QCoreApplication app(argc, argv);
  qDebug() << QString(QObject::tr("A String"));

  return app.exec();
}
```

Invocation of lupdate:
```
$ lupdate main.cpp -ts de.ts
```

lupdate (5.3.1):
```
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1">
<context>
    <name>QObject</name>
    <message>
        <location filename="main.cpp" line="7"/>
        <source>A String</source>
        <translation type="unfinished"></translation>
    </message>
</context>
</TS>
```

lupdate (4.8.6):
```
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.0">
<context>
    <name>QObject</name>
    <message>
        <location filename="main.cpp" line="7"/>
        <source>A String</source>
        <translation type="unfinished"></translation>
    </message>
</context>
</TS>
```

lupdate (4.8.1):
```
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.0">
<context>
    <name>QObject</name>
    <message>
        <location filename="main.cpp" line="7"/>
        <source>A String</source>
        <translation type="unfinished"></translation>
    </message>
</context>
</TS>
```

lupdate (4.6.2):
```
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.0">
<context>
    <name>QObject</name>
    <message>
        <location filename="main.cpp" line="7"/>
        <source>A String</source>
        <translation type="unfinished"></translation>
    </message>
</context>
</TS>
```