scripts: kallsyms: Use %zu to print 'size_t'

Commit f3462aa9 (Kbuild: Handle longer symbols in kallsyms.c) introduced the
following warning on ARM:

scripts/kallsyms.c:121:4: warning: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'size_t' [-Wformat]

Use %zu to print 'size_t'.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>