sim/ppc: Fix check for --sysroot= option

sim/ppc: Fix check for --sysroot= option

Fixes an error reported by Tristan and which can be evidenced by
doing:

        % powerpc-elf-gdb
        (gdb) target sim --sysroot=var
        Invalid option: --sysroot=/var
        [...]

sim/ppc/ChangeLog:

        * psim.c (psim_options): Fix length of comparison when checking
        for --sysroot= option.