Provide (void) argument list for os_boot()'s prototype.

[elric1]

I need this so that I can use -Wmissing-prototypes as clang does not recognise a prototype as valid if the argument list is empty rather than void.

[xchapron-ledger]

Included in https://github.com/LedgerHQ/nanos-secure-sdk/commit/8663123a243894bb1652685e44a787f7df1b9447