Element: [Bug Report] Prefix slot of el-select is not working correctly

[tigerbook73]

Element UI version

2.4.11

OS/Browsers version

Win10/Firefox

Vue version

2.5.21

Reproduction Link

https://jsfiddle.net/qdpc63hw/1/

Steps to reproduce

在el-select中增加prefix slot，增加的内容会和input区域重叠

What is Expected?

prefix slot中的内容和input字段不重叠，大小对齐

What is actually happening?

prefix slot中的内容和input字段重叠，大小不对齐

[island205]

Please write css to fix style issue.