sim: build: move generated headers to built sources

sim: build: move generated headers to built sources

Automake's automatic header deptracking has a bootstrap problem where
it can't detect generated headers when compiling.  We've been handling
that by adding a custom SIM_ALL_RECURSIVE_DEPS variable, but that only
works when building objects recursively in subdirs.  As we move those
out to the top-level, we don't have any recursive steps anymore.  The
Automake approach is to declare those headers in BUILT_SOURCES.

This isn't completely foolproof as the Automake manual documents: it
only activates for `make all`, not `make foo.o`, but that shouldn't be
a huge limitation as it only affects the initial compile.  After that,
rebuilds should work fine.