arm: add slab.h to plat-samsung files for GFP_KERNEL

To fix messages like this:

  CC      arch/arm/plat-samsung/platformdata.o
arch/arm/plat-samsung/platformdata.c: In function 's3c_set_platdata':
arch/arm/plat-samsung/platformdata.c:29: error: 'GFP_KERNEL' undeclared (first use in this function)
arch/arm/plat-samsung/platformdata.c:29: error: (Each undeclared identifier is reported only once
arch/arm/plat-samsung/platformdata.c:29: error: for each function it appears in.)
make[3]: *** [arch/arm/plat-samsung/platformdata.o] Error 1

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>