debuginfod, elfclassify: remove unnecessary header inclusion

`error.h`'s inclusion was centralised into the `system.h` header.  As
the implementation currently includes `system.h` already, the inclusion
of `error.h` is unnecessary.  This prepares for a future portability
change to allow elfutil to build with alternate libc implementations.

Signed-off-by: Saleem Abdulrasool <abdulras@google.com>
(cherry picked from commit d0c72317dcde4c21e88e37dfd865335fc7f0c079)
Change-Id: I37a9b573e447e1382ef8c243f8e4763ddccba3e9
4 files changed