guest: Add generated file guest/guest_init.c to .gitignore

guest/guest_init.c is a generated file, but git doesn't currently
ignore it.  This can be annoying when running git status etc.

This patch adds a suitable .gitignore entry for this file.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
[will: Do the same for guest/guest_pre_init.c]
Signed-off-by: Will Deacon <will.deacon@arm.com>

guest/.gitignore

+guest_pre_init.c
+guest_init.c