fix for install error "PHP Extensions "0" must be loaded.

Since PHP Version 5.4.10 appears an error on during the install process.
![bildschirmfoto 2013-05-04 um 20 40 46](https://f.cloud.github.com/assets/907458/463354/7b3d0e48-b552-11e2-8ccf-a0fe8a3562c9.png)


A simple fix resolves the problem. Change in the File /app/code/core/Mage/Install/etc/config.xml  the Line 
```xml
<pdo_mysql/> to <pdo_mysql>1</pdo_mysql>
```
After the fix I've tested the install process on an older PHP Version (5.3.4) and a newer one (5.4.10), everything worked out perfectly