git: fix static link with openssl

openssl needs -lz when building statically.

Fixes:
http://autobuild.buildroot.net/results/4b3/4b33f3f415776cf43be0910b583d222711a03cad/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>