generate headers for vala extensions
generate headers for vala extensions

This way C code in other extensions can call the vala code.
It's not necessary to add "${CMAKE_BINARY_DIR}/extensions"
to include_directories for them to be found because it's the
current directory when compiling extensions


dokidokivisual/midori,dokidokivisual/midori,dokidokivisual/midori,dokidokivisual/midori,dokidokivisual/midori