setpci: Fix discards const from pointer target

setpci: Fix discards const from pointer target

Since glibc-2.43, and for ISO C23, the function strchr that return
pointer into their input arrays now have definitions as macros that
return a pointer to a const-qualified type when the input argument is
a pointer to a const-qualified type.

The char *e is only used as a pointer to const char **opts, and only
used for comparisons, so declare *e as const to address warning.

Fixes:
    setpci.c: In function 'parse_options':
    setpci.c:513:19: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
      513 |             if (e = strchr(opts, *c))
          |                   ^

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>