main/curl: don't generate completion files when cross-compiling
curl does not support generation of completion files when cross compiling. Since curl is on our boostrap path commit 1a518e3a broke bootstrapping of curl. This commit fixes this by not generating completion files when bootstrapping curl.
See: https://github.com/curl/curl/blob/8e22fc68e7dda43e9f0b6857b1057d0e9131a4b2/scripts/Makefile.am#L39