update-translations: Preserve XML declaration and DOCTYPE

[hebasto]

postprocess_translations currently strips the XML declaration and DOCTYPE from the original files.

For example, in Bitcoin Core on the master branch, you can see bitcoin_en.ts has the headers while bitcoin_fr.ts does not:

$ head -n 4 src/qt/locale/bitcoin_en.ts 
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE TS>
<TS version="2.1" language="en">
<context>

$ head -n 2 src/qt/locale/bitcoin_fr.ts 
<TS version="2.1" language="fr">
<context>

This PR ensures they are manually re-added to prevent any possible Qt compatibility issues.