syscall: avoid unused parameter error if WIFCONTINUED not defined

Fixes https://gcc.gnu.org/PR90614

Change-Id: I4f57817872303b47d9865277e9fb30db26c16abe
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/178997
Reviewed-by: Cherry Zhang <cherryyz@google.com>
1 file changed