Fixes ibus_keyval_name method in c++ binding
Fixes ibus_keyval_name method in c++ binding

Add G_BEGIN_DECLS and G_END_DECLS macros in src/ibuskeys.h

BUG=

Review URL: https://codereview.appspot.com/185850043
Patch from Peng Wu <alexepico@gmail.com>.

j717273419/ibus,ibus/ibus,j717273419/ibus,Keruspe/ibus,Keruspe/ibus,Keruspe/ibus,fujiwarat/ibus,ibus/ibus,fujiwarat/ibus,fujiwarat/ibus,j717273419/ibus,j717273419/ibus,ibus/ibus,Keruspe/ibus,fujiwarat/ibus,ibus/ibus