This fixes the following problem, seen with Boost 1.53.0 on altlinux
when compiling for EDS >= 3.6:
/usr/include/boost/smart_ptr/shared_ptr.hpp: In instantiation of 'typename boost::detail::sp_array_access<T>::type boost::shared_ptr<T>::operator[](std::ptrdiff_t) const [with T = char*; typename boost::detail::sp_array_access<T>::type = void; std::ptrdiff_t = long int]':
src/backends/evolution/EvolutionSyncSource.cpp:163:38: required from here
/usr/include/boost/smart_ptr/shared_ptr.hpp:663:22: error: return-statement with a value, in function returning 'void' [-fpermissive]
make[2]: *** [src/backends/evolution/src_backends_evolution_syncecal_la-EvolutionSyncSource.lo]
The "void" type above is wrong, so it looks like a missing type trait
for the pointer type used in the smart_ptr. PlainGStrArray already had
an at() method to work around such issues, so use it. Not sure why this
one usage of [] slipped through.
dd61308c67