libs/foreignmemory: fix MiniOS build

libs/foreignmemory: fix MiniOS build

Keep the dummy handlers for restrict, map_resource and unmap_resource
for MiniOS, or else the build breaks with:

ld: /home/osstest/build.158759.build-amd64/xen/stubdom/mini-os-x86_64-xenstore/mini-os.o: in function `xenforeignmemory_restrict':
/home/osstest/build.158759.build-amd64/xen/stubdom/libs-x86_64/foreignmemory/core.c:137: undefined reference to `osdep_xenforeignmemory_restrict'
ld: /home/osstest/build.158759.build-amd64/xen/stubdom/mini-os-x86_64-xenstore/mini-os.o: in function `xenforeignmemory_map_resource':
/home/osstest/build.158759.build-amd64/xen/stubdom/libs-x86_64/foreignmemory/core.c:171: undefined reference to `osdep_xenforeignmemory_map_resource'
ld: /home/osstest/build.158759.build-amd64/xen/stubdom/mini-os-x86_64-xenstore/mini-os.o: in function `xenforeignmemory_unmap_resource':
/home/osstest/build.158759.build-amd64/xen/stubdom/libs-x86_64/foreignmemory/core.c:185: undefined reference to `osdep_xenforeignmemory_unmap_resource'
ld: /home/osstest/build.158759.build-amd64/xen/stubdom/mini-os-x86_64-xenstore/mini-os.o: in function `xenforeignmemory_resource_size':
/home/osstest/build.158759.build-amd64/xen/stubdom/libs-x86_64/foreignmemory/core.c:200: undefined reference to `osdep_xenforeignmemory_map_resource'

Fixes: 2b4b33ffe7d67 ('libs/foreignmemory: Implement on NetBSD')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>