Ensure that the .c files that generate_visualc_files.pl enumerates are present
before it runs. Otherwise, depending on the order in which make builds
targets, running `make generated_files` from a fresh checkout could end up
missing `library/error.c` and `library/version_features.c`.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>