perf trace: Show error message when not finding a field used in a filter expression

perf trace: Show error message when not finding a field used in a filter expression

It was there, but as pr_debug(), make it pr_err() so that we can see it
without -v:

  # trace -e syscalls:*lseek --filter="whenc==SET" sleep 1
  "whenc" not found in "syscalls:sys_enter_lseek", can't set filter "whenc==SET"
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-ly4rgm1bto8uwc2itpaixjob@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>