net/ipv4/syncookies.c: remove superfluous header files from syncookies.c

syncookies.c hasn't use any macro or function declared in slab.h and random.h,
Thus, these files can be removed from syncookies.c safely without
affecting the compilation of the net module.

Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>