Fix compilation error with plugin_register_shutdown

Fix compilation error with plugin_register_shutdown

Compiling collectd-4.10.2 on an OpenSuSE 11.3 system causes the
following error:
  snmp.c: In function ‘module_register’:
  snmp.c:1620:3: error: passing argument 1 of ‘plugin_register_shutdown’ discards qualifiers from pointer target type
  plugin.h:275:5: note: expected ‘char *’ but argument is of type ‘const char *’

Convert plugin_register_shutdown's name argument from 'char *' to
'const char *' in order to match the other plugin_register_*
functions.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>