* src/augeas.h: include 'extern "C"' wrapper for C++
* src/augeas.h: include 'extern "C"' wrapper for C++

Fixes https://fedorahosted.org/augeas/ticket/272

Patch from Igor Pashev

dafugg/augeas,raphink/augeas,kumy/augeas,lutter/augeas,GeoffWilliams/augeas,ptoscano/augeas,jtopjian/augeas,lutter/augeas,mlichvar/augeas,kumy/augeas,domcleal/augeas,domcleal/augeas,raphink/augeas,jtopjian/augeas,hercules-team/augeas-do-not-use,mlichvar/augeas,pevalme/augeas,MikaelSmith/augeas,kunkku/augeas,hercules-team/augeas,ptoscano/augeas,hercules-team/augeas,mchf/augeas,jasperla/augeas,dafugg/augeas,kumy/augeas,jjlin/augeas,pevalme/augeas,jasperla/augeas,hercules-team/augeas-do-not-use,kunkku/augeas,kumy/augeas,manandbytes/augeas,jjlin/augeas,mlichvar/augeas,ptoscano/augeas,MikaelSmith/augeas,dafugg/augeas,lutter/augeas,GeoffWilliams/augeas,mchf/augeas,hercules-team/augeas-do-not-use,pevalme/augeas,domcleal/augeas,kunkku/augeas,manandbytes/augeas,jjlin/augeas