bwping : fix build error

fix following error:
/bin/bash: openwrt/build_dir/target-mips_34kc_uClibc-0.9.33.2/bwping-1.4/ipkg-install/usr/sbin: No such file or directory
Makefile:214: recipe for target 'install-sbinPROGRAMS' failed
make[4]: *** [install-sbinPROGRAMS] Error 127
Makefile:495: recipe for target 'install-am' failed
make[3]: *** [install-am] Error 2

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>