selftests/bpf: Fix uninitialized duration warning

Daniel Borkmann reports:

test_progs.c: In function ‘main’:
test_progs.c:81:3: warning: ‘duration’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   printf("%s:PASS:%s %d nsec\n", __func__, tag, duration);\
   ^~~~~~
test_progs.c:1706:8: note: ‘duration’ was declared here
  __u32 duration;
        ^~~~~~~~

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>