Add the missing header for `errno` variable in `checks.cc`
Add the missing header for `errno` variable in `checks.cc`

Variable `LAST_SYSTEM_ERROR` was introduced in https://webrtc-review.googlesource.com/c/src/+/32780.
It seems to be the same codeblock in `physicalsocketserver.cc`, only difference is it did not
include the header <errno.h>.

Also, probably a good idea to make the include conditional.

Bug: None
Change-Id: I3241dd83be4a248c6c1db2fab8f924a185e354cb
Reviewed-on: https://webrtc-review.googlesource.com/45864
Reviewed-by: Niels Moller <0057859e47d61d90529d0f66d9ef692cefedb46a@webrtc.org>
Reviewed-by: Tommi <74a97831ec830b6e3846c6faa50560947ae01c68@webrtc.org>
Commit-Queue: Niels Moller <0057859e47d61d90529d0f66d9ef692cefedb46a@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#21841}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc