configure: fix curl_off_t check's include order

- Prepend srcdir include path instead of append.

Prior to this change it was possible that during the check for the size
of curl_off_t the include path of a user's already installed curl could
come before the include path of the to-be-built curl, resulting in the
system.h of the former being incorrectly included for that check.

Closes https://github.com/curl/curl/pull/1870

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