libhx509 is not built according to the same export and calling conventions
on Windows as the other libraries. This change declares and applies
HX509_LIB_FUNCTION, HX509_LIB_NORETURN_FUNCTION, HX509_LIB_CALL and
HX509_LIB_VARIABLE to lib/hx509.
As a result of this change the calling convention for exported functions
will be __stdcall instead of __cdecl.
Change-Id: Ibc3f05e8088030ef7d13798f1d9c9b190bc57797