build-uefi.sh: fix repo tool issue with fetching git submodules

Version 2.15 of the repo tool breaks the fetching of git submodules. The
patch that fixes this issue is
https://gerrit.googlesource.com/git-repo/+/d177609cb0283e41e23d4c19b94e17f42bdbdacb


Until this fix is made available in repo tool, explicity fetch the
submodules before the build.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I73c6221322f2e27dc56a40e1b114f2c1d536d9cb