x86/boot: Fix build dependenices for reloc.c

x86/boot: Fix build dependenices for reloc.c

c/s 201f852eaf added start_info.h and kconfig.h to reloc.c, but only updated
start_info.h in RELOC_DEPS.

This causes reloc.c to not be regenerated when Kconfig changes.  It is most
noticeable when enabling CONFIG_PVH and finding the resulting binary crash
early with:

  (d9) (XEN)
  (d9) (XEN) ****************************************
  (d9) (XEN) Panic on CPU 0:
  (d9) (XEN) Magic value is wrong: c2c2c2c2
  (d9) (XEN) ****************************************
  (d9) (XEN)
  (d9) (XEN) Reboot in five seconds...
  (XEN) d9v0 Triple fault - invoking HVM shutdown action 1

Reported-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>