x86/retpoline: Move the retpoline thunk declarations to nospec-branch.h

Because it makes no sense to split the retpoline gunk over multiple
headers.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.106290934@infradead.org