examples/fips_validation: fix link to libjansson

examples/fips_validation: fix link to libjansson

When compiling this example out of DPDK, linking the executable fails
with:

  ## Building fips_validation
  /usr/bin/ld: /tmp/ccQjeHBg.o: in function `fips_test_init':
  fips_validation.c:(.text+0x7ab): undefined reference to `json_loadf'
  /usr/bin/ld: /tmp/ccQjeHBg.o: in function
     `fips_test_parse_one_json_vector_set':
  fips_validation.c:(.text+0xc2e): undefined reference to `json_object_get'
  /usr/bin/ld: fips_validation.c:(.text+0xc36): undefined reference to
     `json_string_value'
  /usr/bin/ld: /tmp/ccQjeHBg.o: in function `fips_test_parse_one_json_group':
  fips_validation.c:(.text+0xd00): undefined reference to `json_object_get'
  /usr/bin/ld: fips_validation.c:(.text+0xd14): undefined reference to
     `json_integer_value'
  ...

Code in an example can't rely on RTE_HAS_JANSSON, because it only
indicates that the jansson library was available at the time of dpdk
compilation.

Prefer a local build flag (like what is done in vm_power_manager).
And add linking to libjansson, if available.

Fixes: f556293fd58e ("examples/fips_validation: add JSON info to header")
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Gowrishankar Muthukrishnan <gmuthukrishn@marvell.com>