maint: fix ioctls_sym.sh premature termination

maint: fix ioctls_sym.sh premature termination

When the list of headers do not fit into the command line, xargs splits
it into several lists and then passes them to grep.  If no headers from
a list match the grep pattern, grep exits with a non-zero status that
causes xargs to exit with a non-zero status, too.

* maint/ioctls_sym.sh: Use "find -exec +" instead of xargs,
ignore its exit status.