Fix avr_global_logger_get() decl  to not warn

When using sim_avr.h with a project that uses -Wstrict-prototypes, you
will get a warning that the prototype for avr_global_logger_get() is not
valid. Functions that take no args must explicitly use void to not warn.
Merge pull request #45 from cardoe/prototype-warning

Fix avr_global_logger_get() decl  to not warn.