Fix: leak of lttng_elf_shdr in lttng-elf.c

Most lttng_elf_shdr uses in lttng-elf.c end-up leaking the header.
Since those headers instances are always short-lived and relatively
small, these leaks are fixed by not using dynamic allocations and
instead using automatic storage.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>