Remove bashism in lttng-syscalls-generate-headers.sh

Options to echo are not portable. In particular, the 'echo -e' option is
implemented by some shells, including bash, to expand escape sequences.
However, dash is one of the other family of shells that instead expands
escape sequences by default.

The printf command is portable and much more reliable.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>