perf trace x86_arch_prctl: Address shellcheck warnings about local variables

Running shellcheck on x86_arch_prctl.sh generates below warning:

  In ./tools/perf/trace/beauty/x86_arch_prctl.sh line 10:
      	local idx=$1
        ^-------^ SC3043 (warning): In POSIX sh, 'local' is undefined.

  In ./tools/perf/trace/beauty/x86_arch_prctl.sh line 11:
	local prefix=$2
        ^----------^ SC3043 (warning): In POSIX sh, 'local' is undefined.

   In ./tools/perf/trace/beauty/x86_arch_prctl.sh line 12:
	local first_entry=$3
        ^---------------^ SC3043 (warning): In POSIX sh, 'local' is undefined.

Fix this by removing local since these are variables used only in
specific function

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230709182800.53002-21-atrajeev@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>