GIT-INFO: suggest using autoreconf instead of buildconf following 85868537d

[starrify]

A minor change to resolve the conflicts in documentations.

Someone that is new to this project may, as I earlier did myself, start from docs/INSTALL.md and observe these lines:

curl/docs/INSTALL.md

Lines 22 to 26 in 1763ace

	## Building from git
	If you get your code off a git repository instead of a release tarball, see
	the `GIT-INFO` file in the root directory for specific instructions on how to
	proceed.

Then in GIT-INFO there are:

curl/GIT-INFO

Lines 13 to 18 in 1763ace

	To build in environments that support configure, after having extracted
	everything from git, do this:
	./buildconf
	./configure
	make

However ./buildconf has been obsoleted as part of 8586853:

curl/buildconf

Lines 3 to 4 in 1763ace

	echo "*** Do not use buildconf. Instead, just use: autoreconf -fi" >&2
	exec ${AUTORECONF:-autoreconf} -fi "${@}"

Therefore this PR attempts to update GIT-INFO to have the recommended approach.

[bagder]

Thanks!