x86, boot: add new generated files to the appropriate .gitignore files

git status complains of untracked (generated) files in arch/x86/boot..

# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       ../../arch/x86/boot/compressed/mkpiggy
#       ../../arch/x86/boot/compressed/piggy.S
#       ../../arch/x86/boot/compressed/vmlinux.lds
#       ../../arch/x86/boot/voffset.h
#       ../../arch/x86/boot/zoffset.h

..so adjust .gitignore files accordingly.
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/boot/.gitignore

@@ -3,6 +3,8 @@ bzImage
 cpustr.h
 mkcpustr
 offsets.h
+voffset.h
+zoffset.h
 setup
 setup.bin
 setup.elf

arch/x86/boot/compressed/.gitignore

 relocs
 vmlinux.bin.all
 vmlinux.relocs
+vmlinux.lds
+mkpiggy
+piggy.S