perf arch x86: Address shellcheck warnings about unused variables in syscalltbl.sh

Running shellcheck on syscalltbl.sh generates below warning:

In ./tools/perf/arch/x86/entry/syscalls/syscalltbl.sh line 27:
while read nr abi name entry compat; do
^-^ SC2034 (warning): abi appears unused. Verify use (or export if used externally).
^----^ SC2034 (warning): compat appears unused. Verify use (or export if used externally).

These variables are intentionally unused since they are needed to parse
through the output. Use "_" as a prefix for these throw away variables.

Signed-off-by: Athira Rajeev <[e-mail]>
Acked-by: Ian Rogers <[e-mail]>
Cc: Disha Goel <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Kajol Jain <[e-mail]>
Cc: Madhavan Srinivasan <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/arch/x86/entry/syscalls/syscalltbl.sh

···
 grep '^[0-9]' "$in" | sort -n > $sorted_table
 
 max_nr=0
-while read nr abi name entry compat; do
+while read nr _abi name entry _compat; do
     if [ $nr -ge 512 ] ; then # discard compat sycalls
         break
     fi