samples/bpf: fix warnings in xdp_monitor_user

Make local functions static to fix

  HOSTCC  samples/bpf/xdp_monitor_user.o
samples/bpf/xdp_monitor_user.c:64:7: warning: no previous prototype for ‘gettime’ [-Wmissing-prototypes]
 __u64 gettime(void)
       ^~~~~~~
samples/bpf/xdp_monitor_user.c:209:6: warning: no previous prototype for ‘print_bpf_prog_info’ [-Wmissing-prototypes]
 void print_bpf_prog_info(void)
      ^~~~~~~~~~~~~~~~~~~
Fixes: 3ffab546 ("samples/bpf: xdp_monitor tool based on tracepoints")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>