Set dependency against only object

[lissyx]

Not doing so may result in build failure:

$ sudo make signed
make -C /lib/modules/6.14.0-28-generic/build M= modules
make[1] : on entre dans le répertoire « /usr/src/linux-headers-6.14.0-28-generic »

make[3]: ***  Aucune règle pour fabriquer la cible « arch/x86/entry/syscalls/syscall_32.tbl », nécessaire pour « arch/x86/include/generated/uapi/asm/unistd_32.h ». Arrêt.
make[2]: *** [arch/x86/Makefile:277: archheaders] Error 2
make[1]: *** [Makefile:251: __sub-make] Error 2
make[1] : on quitte le répertoire « /usr/src/linux-headers-6.14.0-28-generic »
make: *** [Makefile:4: all] Error 2

[GitMensch]

There is no signed target in the Makefile (and never was in master), so I guess this PR can be closed.
If the signed target is added then the dependency should be configured like here.