elfstructs: add relocation defines for i386

elfstructs: add relocation defines for i386

Those are need by the rombios relocation code in hvmloader. Fixes the
following build error:

32bitbios_support.c: In function 'relocate_32bitbios':
32bitbios_support.c:130:18: error: 'R_386_PC32' undeclared (first use in this function); did you mean 'R_X86_64_PC32'?
             case R_386_PC32:
                  ^~~~~~~~~~
                  R_X86_64_PC32
32bitbios_support.c:130:18: note: each undeclared identifier is reported only once for each function it appears in
32bitbios_support.c:134:18: error: 'R_386_32' undeclared (first use in this function)
             case R_386_32:
                  ^~~~~~~~

Only add the two defines that are actually used, which seems to match
what we do for amd64.

Fixes: 81b2b328a26c1b ('hvmloader: use Xen private header for elf structs')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>