scripts: fix offset.h header guard

Using __hash__() is unpredictable and produces different results for
different python processes. This has been causing unnecessary rebuilds
due to changing header files at the top of the dependency tree.

Just use _GEN_OFFSETS_H_.

Jira: ZEP-2457
Signed-off-by: Anas Nashif <anas.nashif@intel.com>