accel/amdxdna: Add missing include linux/slab.h

When compiling without CONFIG_IA32_EMULATION, there can be some errors:

drivers/accel/amdxdna/amdxdna_mailbox.c: In function ‘mailbox_release_msg’:
drivers/accel/amdxdna/amdxdna_mailbox.c:197:2: error: implicit declaration
of function ‘kfree’.
  197 |  kfree(mb_msg);
      |  ^~~~~
drivers/accel/amdxdna/amdxdna_mailbox.c: In function ‘xdna_mailbox_send_msg’:
drivers/accel/amdxdna/amdxdna_mailbox.c:418:11: error:implicit declaration
of function ‘kzalloc’.
  418 |  mb_msg = kzalloc(sizeof(*mb_msg) + pkg_size, GFP_KERNEL);
      |           ^~~~~~~

Add the missing include.

Fixes: b87f920b ("accel/amdxdna: Support hardware mailbox")
Signed-off-by: Su Hui <suhui@nfschina.com>
Reviewed-by: Lizhi Hou <lizhi.hou@amd.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250211015354.3388171-1-suhui@nfschina.com