perf symbols: Use (long) for iterator for bfd symbols

perf symbols: Use (long) for iterator for bfd symbols

GCC (GCC) 8.4.0 20200304 fails to build perf with:
: util/symbol.c: In function 'dso__load_bfd_symbols':
: util/symbol.c:1626:16: error: comparison of integer expressions of different signednes
:   for (i = 0; i < symbols_count; ++i) {
:                 ^
: util/symbol.c:1632:16: error: comparison of integer expressions of different signednes
:    while (i + 1 < symbols_count &&
:                 ^
: util/symbol.c:1637:13: error: comparison of integer expressions of different signednes
:    if (i + 1 < symbols_count &&
:              ^
: cc1: all warnings being treated as errors

It's unlikely that the symtable will be that big, but the fix is an
oneliner and as perf has CORE_CFLAGS += -Wextra, which makes build to
fail together with CORE_CFLAGS += -Werror

Fixes: eac9a4342e54 ("perf symbols: Try reading the symbol table with libbfd")
Signed-off-by: Dmitry Safonov <dima@arista.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Jacek Caban <jacek@codeweavers.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Remi Bernon <rbernon@codeweavers.com>
Link: http://lore.kernel.org/lkml/20210209145148.178702-1-dima@arista.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>