utils: move parse_percent() to tc_util

utils: move parse_percent() to tc_util

As parse_percent() is used only in tc.

This reduces ip, bridge and genl binaries size:

$ bloat-o-meter -t bridge/bridge bridge/bridge.new
add/remove: 0/1 grow/shrink: 0/0 up/down: 0/-109 (-109)
Total: Before=50973, After=50864, chg -0.21%

$ bloat-o-meter -t genl/genl genl/genl.new
add/remove: 0/1 grow/shrink: 0/0 up/down: 0/-109 (-109)
Total: Before=30298, After=30189, chg -0.36%

$ bloat-o-meter ip/ip ip/ip.new
add/remove: 0/1 grow/shrink: 0/0 up/down: 0/-109 (-109)
Total: Before=674164, After=674055, chg -0.02%

Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: David Ahern <dsahern@gmail.com>