build: fix missing SOURCE_VERSION variable
The recipe Download/git-kernel uses DownloadMethod/git
which now requires a definition of SOURCE_VERSION instead of VERSION
due to Validate/git being used to check for the variables.
Rename the variable as intended to match with the others
that were renamed in the referenced commit.
This fixes the following Makefile parse error
when downloading a specific kernel repository version
when configured with the CONFIG_KERNEL_GIT_CLONE_URI option:
Makefile:19: *** Download/git-kernel is missing the SOURCE_VERSION field.. Stop.
Fixes: 9fc79e2e2 ("download: don't overwrite VERSION variable")
Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/15858
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>