Remove unnecessary includes in lvm_base.c.

Remove unnecessary includes in lvm_base.c.

We would like to declare our handles pv_t, vg_t, and lv_t in
the external library header lvm.h.  However, these are already
defined in metadata-exported.h for the use of some of the
in-progress liblvm APIs.  Thus, we cannot both define
them in lvm.h and include metadata-exported.h in the external
library C files.  We could use preprocessor tricks (#ifndef)
but for now we just avoid the include.