cheri: malloc: bump OBSTACK_INTERFACE_VERSION

cheri: malloc: bump OBSTACK_INTERFACE_VERSION

The obstack version should have been bumped after

  commit 2fd4de4b15a66f821057af90714145d2c034a609
  [BZ #321]

which changed a ptrdiff_t struct member into a union of ptrdiff_t and a
pointer and thus changed the ABI on targets where ptrdiff_t and pointer
have different size or alignment. This affects CHERI targets.

Old versions of obstack are used in the wild and conflict with the one
provided by glibc so at least on CHERI targets it has to be bumped.
On other targets we don't bump the version as it changes compile time
behaviour (the ABI remains backward compatible either way).