include header that declares __syscall_ret where it's defined

in general, we aim to always include the header that's declaring a
function before defining it so that the compiler can check that
prototypes match.

additionally, the internal syscall.h declares __syscall_ret with a
visibility attribute to improve code generation for shared libc (to
prevent gratuitous GOT-register loads). this declaration should be
visible at the point where __syscall_ret is defined, too, or the
inconsistency could theoretically lead to problems at link-time.