pppd: Fix building session.c with GCC 15 (#553)

pppd: Fix building session.c with GCC 15 (#553)

Fixed building with GCC 15 which defaults to C23
and find conflicting declration of getspnam() here
with the one provided by shadow.h (extern struct spwd *getspnam (const char *__name);)

Fixes
../../ppp-2.5.2/pppd/session.c: In function 'session_start':
../../ppp-2.5.2/pppd/session.c:185:18: error: conflicting types for 'getspnam'; have 'struct spwd *(void)'
  185 |     struct spwd *getspnam();
      |                  ^~~~~~~~

Signed-off-by: Khem Raj <raj.khem@gmail.com>