package/libcurl: fix libgsasl support

Contrary to the helptext
https://github.com/curl/curl/blob/master/configure.ac#L152

the configure option is called --with-libgsasl:
https://github.com/curl/curl/blob/master/configure.ac#L1989



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>