meta_data.c: fix a build warning

meta_data.c: fix a build warning

make[3]: Entering directory '/home/ruben/src/collectd/src/daemon'
  CC       meta_data.lo
meta_data.c:728:12: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
    actual = e->value.mv_boolean ? "true" : "false";
           ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
  CCLD     libmetadata.la
  CC       collectd-meta_data.o
meta_data.c:728:12: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
    actual = e->value.mv_boolean ? "true" : "false";
           ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.