i2c-eg20t: include slab.h for memory allocations

Fixes (with v2.6.38-rc3/parisc/parisc-allmodconfig):
  src/drivers/i2c/busses/i2c-eg20t.c:720: error: implicit declaration of function 'kzalloc'
  src/drivers/i2c/busses/i2c-eg20t.c:790: error: implicit declaration of function 'kfree'

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Tomoya MORINAGA <tomoya-linux@dsn.okisemi.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>