kernel-devsrc: powerpc: add missed dependency for modules_prepare

kernel-devsrc: powerpc: add missed dependency for modules_prepare

There is an issue during 'make modules_prepare' execution for PPC_85xx:

arch/powerpc/kernel/asm-offsets.c:58:10: fatal error: head_booke.h: No such file or directory
   58 | #include "head_booke.h"
      |          ^~~~~~~~~~~~~~
compilation terminated.
make[1]: *** [scripts/Makefile.build:118: arch/powerpc/kernel/asm-offsets.s] Error 1

asm-offests.c has dependency on head_booke.h so add it to kernel-devsrc package

Signed-off-by: Denys Zagorui <dzagorui@cisco.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>