src: Update macro guard name

Change macro guard name for `include/internal/types.h` from
`__TYPES_H__` to `__INTERNAL_TYPES_H__`. This prevents it from clashing
with the macro guard for `include/sys/types.h`.

Unless this change is applied, the inclusion of `sys/types.h` will not
lead to the inclusion of `internal/types.h`, since the macro guard
`__TYPES_H__` is used in both.

Signed-off-by: Razvan Deaconescu <razvan.deaconescu@upb.ro>