Fix implicit declaration of function 'malloc'

Fix implicit declaration of function 'malloc'

First observed on Gentoo Linux with GCC 14. This is due to GCC 14
enabling -Werror=implicit-function-declaration by default.
Thus resulting in errors such as:

openconnect-internal.h: In function 'alloc_pkt':
openconnect-internal.h:911:27: error: implicit declaration of function 'malloc' [-Werror=implicit-function-declaration]
  911 |         struct pkt *pkt = malloc(alloc_len);
      |                           ^~~~~~

Plese refer gentoo bug: https://bugs.gentoo.org/923173
Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>