define-syscall: Add no_std attribute

[febo]

Problem

Currently the define-syscall crate is no_std "friendly" but does not have the crate attribute. This indirectly brings the std at link stage when trying to compile no_std programs:

error[E0152]: found duplicate lang item `panic_impl`
 --> src/lib.rs:9:1
  |
9 | pinocchio::nostd_panic_handler!();
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: the lang item is first defined in crate `std` (which `solana_define_syscall` depends on)

Solution

Add the #![no_std] attribute to the crate.

[joncinque]

Looks great to me!