KVM: selftests: Add EXTRA_CFLAGS in top-level Makefile

We add EXTRA_CFLAGS to the common CFLAGS of top-level Makefile which will
allow users to pass additional compile-time flags such as "-static".

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-and-tested-by: Atish Patra <atishp@rivosinc.com>
Reviewed-and-tested-by: Sean Christopherson <seanjc@google.com>