Fix -Wsign-compare warning

[23/236] Compiling C object 'lib/76b5a...isc@sha/contrib_gvdb_gvdb-reader.c.o'.
../../../../Projects/epiphany/lib/contrib/gvdb/gvdb-reader.c: In function ‘gvdb_table_get_names’:
../../../../Projects/epiphany/lib/contrib/gvdb/gvdb-reader.c:428:27: warning: comparison of integer expressions of different signedness: ‘guint32’ {aka ‘unsigned int’} and ‘gint’ {aka ‘int’} [-Wsign-compare]
  428 |           else if (parent < n_names && names[parent] != NULL)
      |                           ^

To fix this, we have to change n_names to guint, and then also change
the types of everything it's compared against. This seems to be safe
since none of these should ever be negative.