lib/{e2p,ss}: remove manual declarations of errno

lib/{e2p,ss}: remove manual declarations of errno

As per 'man 3 errno':

    On some ancient systems, <errno.h> was not present or did not
    declare errno, so that it was necessary to declare errno manually
    (i.e., extern int errno).   **Do not do this**.  It long ago ceased
    to be necessary, and it will cause problems with modern versions of
    the C library.

One of the platforms it causes a problem on is Windows:

    In file included from fgetversion.c:28:
    fgetversion.c: In function ‘fgetversion’:
    fgetversion.c:68:20: warning: ‘_errno’ redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
       68 |         extern int errno;
          |                    ^~~~~

Just remove these obsolete manual declarations of errno.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>