rpmbuild: Try to initialize submodules prior running the build

In case someone clones bare SPDK repo and attempts to execute the
RPM build, it will fail due to missing submodules. Since it's cheap,
just make sure submodules are in place before the build starts.

Signed-off-by: Michal Berger <michal.berger@intel.com>
Change-Id: I910949ae5fc5ef19c40f4abfc6f39ebb3fe8929a
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/16307


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Kamil Godzwon <kamilx.godzwon@intel.com>