libc: newlib: Fix declare a type conflict

Fix a variable declaration type conflict:

libc-hooks.c:92:16: error: conflicting types for '_heap_sentry'
   92 |   extern void *_heap_sentry;
soc.h:78:13: note: previous declaration of '_heap_sentry' was here

Fixes #44926

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>