Address CRAN check issues present on v1.0.0

[coatless]

https://cran.r-project.org/web/checks/check_results_sitmo.html

Add src/init.c via tools::package_native_routine_registration_skeleton(".")

Version: 1.0.0
Check: compiled code
Result: NOTE
File ‘sitmo/libs/sitmo.so’:
Found no calls to: ‘R_registerRoutines’, ‘R_useDynamicSymbols’

It is good practice to register native routines and to disable symbol
search.

See ‘Writing portable packages’ in the ‘Writing R Extensions’ manual. 

Flavors: r-devel-linux-x86_64-debian-clang, r-devel-linux-x86_64-debian-gcc, r-patched-linux-x86_64, r-release-linux-x86_64