s390/purgatory: update .gitignore

Since commit 4c0f032d ("s390/purgatory: Omit use of bin2c"),
kexec-purgatory.c is not generated.

purgatory and purgatory.lds are generated files, so should be ignored
by git.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>