Prevent Visual Studio from generating PDB files

[aisouard]

PDB files shouldn't be generated under Release mode. Debugging info should be embedded directly inside the resulting lib file, otherwise it will output the following message during link:

webrtc.lib(webrtcsdp.obj) : warning LNK4099: PDB 'libjingle_peerconnection_cc.pdb'
  was not found with 'webrtc.lib(webrtcsdp.obj)' or at 'C:\projects\node-webrtc\build\
  Release\libjingle_peerconnection_cc.pdb'; linking object as if no debug info
  [C:\projects\node-webrtc\build\webrtc.vcxproj]