Don't fail script if folder already exists
Don't fail script if folder already exists

Caused build failure when run in Jenkins:

```
+ cd /var/lib/jenkins/jobs/build-webrtc/workspace/android/webrtc/libjingle_peerconnection_builds/Debug
+ mkdir res
mkdir: cannot create directory ‘res’: File exists
Build step 'Execute shell' marked build as failure
Finished: FAILURE
```
MindsIgnited/webrtc-build-scripts,pristineio/webrtc-build-scripts,sundeepdev/webrtc-build-scripts,liushimeng/webrtc-build-scripts,czhao/webrtc-build-scripts,contra/webrtc-build-scripts,zero1zero/webrtc-build-scripts,zzhhui/webrtc-build-scripts,act262/webrtc-build-scripts,huynguyen/webrtc-build-scripts,hareshsandeep/webrtc-build-scripts,hareshsandeep/webrtc-build-scripts,zero1zero/webrtc-build-scripts,zzhhui/webrtc-build-scripts,contra/webrtc-build-scripts,imReker/webrtc-build-scripts,pristineio/webrtc-build-scripts,czhao/webrtc-build-scripts,adozenlines/webrtc-build-scripts,ShengQiangLiu/webrtc-build-scripts,nehaysingh/webrtc-build-scripts,adozenlines/webrtc-build-scripts,liushimeng/webrtc-build-scripts,GabrielCastro/webrtc-build-scripts,act262/webrtc-build-scripts,hareshsandeep/webrtc-build-scripts,sundeepdev/webrtc-build-scripts,lulufei/webrtc-build-scripts,sundeepdev/webrtc-build-scripts,ShengQiangLiu/webrtc-build-scripts,shelsonjava/webrtc-build-scripts,ShengQiangLiu/webrtc-build-scripts,aditya-agarwal/webrtc-build-scripts,shelsonjava/webrtc-build-scripts,pristineio/webrtc-build-scripts,adozenlines/webrtc-build-scripts,MindsIgnited/webrtc-build-scripts,lulufei/webrtc-build-scripts,liushimeng/webrtc-build-scripts,shelsonjava/webrtc-build-scripts,GabrielCastro/webrtc-build-scripts,MindsIgnited/webrtc-build-scripts,zero1zero/webrtc-build-scripts,huynguyen/webrtc-build-scripts,nehaysingh/webrtc-build-scripts,huynguyen/webrtc-build-scripts,act262/webrtc-build-scripts,aditya-agarwal/webrtc-build-scripts,zzhhui/webrtc-build-scripts,imReker/webrtc-build-scripts,aditya-agarwal/webrtc-build-scripts,nehaysingh/webrtc-build-scripts,lulufei/webrtc-build-scripts,contra/webrtc-build-scripts,GabrielCastro/webrtc-build-scripts,czhao/webrtc-build-scripts,imReker/webrtc-build-scripts