api: add os.(*File).SyscallConn to go1.12.txt

Fixes #29507

Change-Id: I8cf52e4b89fd28126f252757260d07a31d9dad61
Reviewed-on: https://go-review.googlesource.com/c/155933Reviewed-by: Katie Hockman <katie@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

api/go1.12.txt

@@ -37,6 +37,7 @@ pkg net/http, const StatusTooEarly ideal-int
 pkg net/http, method (*Client) CloseIdleConnections()
 pkg os, const ModeType = 2401763328
 pkg os, func UserHomeDir() (string, error)
+pkg os, method (*File) SyscallConn() (syscall.RawConn, error)
 pkg os, method (*ProcessState) ExitCode() int
 pkg os/exec, method (ExitError) ExitCode() int
 pkg reflect, method (*MapIter) Key() Value