runtime/cgo: fix build on freebsd/arm

This CL is in preparation to make cgo work on freebsd/arm.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/60500044

src/pkg/runtime/cgo/gcc_freebsd_arm.c

@@ -4,6 +4,7 @@
 
 #include <sys/types.h>
 #include <machine/sysarch.h>
+#include <sys/signalvar.h>
 #include <pthread.h>
 #include <signal.h>
 #include <string.h>