src/libcgroup-internal: drop dirent/mntent includes

src/libcgroup-internal: drop dirent/mntent includes

src/libcgroup-internal.h includes <dirent.h> and <mntent.h>, but only
uses pointer types in function declarations.

Replace the heavy includes with forward declarations:
- add struct dirent;
- add struct mntent;
- remove <dirent.h> and <mntent.h> from the internal header

This trims transitive header dependencies and reduces compile noise,
with no functional change.

Build tested with:
  make -j4

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka (Oracle) <tom.hromatka@gmail.com>