telemetry: fix build

telemetry: fix build

Build error:
../lib/librte_telemetry/rte_telemetry.c:558:28:
  error: comparison of unsigned expression < 0 is always false
  [-Werror,-Wtautological-compare]

Build error not observed in default make build because telemetry library
disabled by default but easier to reproduce via meson.

Fixing by converting unsigned variables to signed.

Fixes: 0fe3a37924d4 ("telemetry: format json response when sending stats")
Fixes: 4080e46c8078 ("telemetry: support global metrics")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Kevin Laatz <kevin.laatz@intel.com>