package/libcurl: bump to version 7.71.1

package/libcurl: bump to version 7.71.1

This release fixes build with BR2_PACKAGE_LIBCURL_PROXY_SUPPORT disabled
and mbedtls enabled.

Add reference to upstream tarball signature.

Fixes:
http://autobuild.buildroot.net/results/f32b6ab927560839cacaa1b9e6b64ced92b9ffe3/
http://autobuild.buildroot.net/results/566f0db496f6d1feefd9d3e6b6955a2539670735/
http://autobuild.buildroot.net/results/19de1111318aea863118c9b0b44dc282f011918f/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>