[build] [llcpp] Allow standalone usage of llcpp without C bindings.

Previously, the GN templates for llcpp incorrectly assumed the existence
of C bindings, causing missing symbols error at link time when the C
bindings is not present. This change incorporates the necessary
generated files into the llcpp target.

Test: Add fuchsia-io:llcpp to a target that does not import
fuchsia-io: c, and builds. E.g. this case
https: //fuchsia-review.googlesource.com/c/fuchsia/+/273533
Change-Id: I5469b24698df028701cf69d6e601ebe957c99b70