Fix gdx-bullet swigging on case-sensitive systems

Error part of ant build script output:
```
gen:
     [echo] Swigging
     [exec] /home/alex/libgdx/extensions/gdx-bullet/jni/swig/extras/./serialize/gdxBulletSerialize.i:40: Error: Unable to find 'BulletWorldImporter/btWorldImporter.h'
     [exec] /home/alex/libgdx/extensions/gdx-bullet/jni/swig/extras/./serialize/gdxBulletSerialize.i:41: Error: Unable to find 'BulletWorldImporter/btBulletWorldImporter.h'
     [exec] Result: 1
```
`src/extras/serialize/` doesn't exists, `src/extras/Serialize/` do.