gflags: put hash in .hash file
github archives should be stable now.
Fixes:
http://autobuild.buildroot.net/results/ac9/ac92ec88c9e26df579a5e44f58c34a3e4dd5b2b0/
http://autobuild.buildroot.net/results/79c/79cecd4fa98bdad5b5bdc57725d8cb8118b1cf2e/
http://autobuild.buildroot.net/results/bb2/bb2e04a643784beb78f38c016a1c621786b575fe/
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>