Fix warning for duplicate declaration of `index`

Rename the variable hat identifies values within the sql query with
placeholders to `ind` from `index`. This fixes the following warning:

/home/vaibhav/lwan/freegeoip/freegeoip.c: In function
‘text_column_helper’:
/home/vaibhav/lwan/freegeoip/freegeoip.c:232:44: warning: declaration of
‘index’ shadows a global declaration [-Wshadow]