s390: reintroduce expoline dependence to scripts

Expolines depend on scripts/basic/fixdep. And build of expolines can now
race with the fixdep build:

 make[1]: *** Deleting file 'arch/s390/lib/expoline/expoline.o'
 /bin/sh: line 1: scripts/basic/fixdep: Permission denied
 make[1]: *** [../scripts/Makefile.build:385: arch/s390/lib/expoline/expoline.o] Error 126
 make: *** [../arch/s390/Makefile:166: expoline_prepare] Error 2

The dependence was removed in the below Fixes: commit. So reintroduce
the dependence on scripts.

Fixes: a0b0987a ("s390/nospec: remove unneeded header includes")
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Cc: stable@vger.kernel.org
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: linux-s390@vger.kernel.org
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230316112809.7903-1-jirislaby@kernel.orgSigned-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/Makefile

@@ -162,7 +162,7 @@ vdso_prepare: prepare0
 
 ifdef CONFIG_EXPOLINE_EXTERN
 modules_prepare: expoline_prepare
-expoline_prepare:
+expoline_prepare: scripts
 	$(Q)$(MAKE) $(build)=arch/s390/lib/expoline arch/s390/lib/expoline/expoline.o
 endif
 endif