elfutils: use https protocol for submodule

Using git:// protocol for submodules quite often causes issues in
corporate setting where development machines are behind firewalls. For
http/https usually some transparent proxy would be provided, while git
protocol isn't proxied as nicely.

Let's avoid unnecessary complications and use https:// protocol for
elfutils submodule.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

.gitmodules

@@ -3,7 +3,7 @@
 	url = https://github.com/libbpf/libbpf.git
 [submodule "elfutils"]
 	path = elfutils
-	url = git://sourceware.org/git/elfutils.git
+	url = https://sourceware.org/git/elfutils.git
 [submodule "zlib"]
 	path = zlib
 	url = https://github.com/madler/zlib.git