git: fix config option used to detect libcurl availability

BR2_PACKAGE_CURL is currently used by the git package to find out if
libcurl is available. While indeed BR2_PACKAGE_CURL indicates that
libcurl is available, it is not the most appropriate option for this:
BR2_PACKAGE_LIBCURL is better. BR2_PACKAGE_LIBCURL indicates that
libcurl is available, while BR2_PACKAGE_CURL indicates that both
libcurl and the curl program are available. Only the former is needed
by Git.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>