[POWERPC] Fix build failure on legacy iSeries

Include <asm/iseries/hv_call.h> in arch/powerpc/mm/stab.c to fix the
following compile error (found with randconfig):

  CC      arch/powerpc/mm/stab.o
arch/powerpc/mm/stab.c: In function "stab_initialize":
arch/powerpc/mm/stab.c:282: error: implicit declaration of function "HvCall1"
arch/powerpc/mm/stab.c:282: error: "HvCallBaseSetASR" undeclared (first use in this function)
arch/powerpc/mm/stab.c:282: error: (Each undeclared identifier is reported only once
arch/powerpc/mm/stab.c:282: error: for each function it appears in.)
make[1]: *** [arch/powerpc/mm/stab.o] Error 1
make: *** [arch/powerpc/mm] Error 2

Signed-off-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>