Fix const-correctness warning in linux.c

Addresses issue #4814 by adding explicit cast when assigning string
literal "unknown" to mutable char* pointer. The cast is safe because
the string is only read from, never modified.

This eliminates compiler warnings when building with strict const-correctness
flags like -Wwrite-strings.

src/unix/linux.c

@@ -1739,7 +1739,7 @@ int uv_cpu_info(uv_cpu_info_t** ci, int* count) {
     unsigned model;
   };
   FILE* fp;
-  char* p;
+  const char* p;
   int found;
   int n;
   unsigned i;