BUILD/MINOR: cpuset freebsd build fix

Add missing strings.h header. Required for ffsl definition used by
CPU_FFS macro.

This must be backported up to 2.4.
diff --git a/include/haproxy/cpuset-t.h b/include/haproxy/cpuset-t.h
index 0d5b6c8..541fb75 100644
--- a/include/haproxy/cpuset-t.h
+++ b/include/haproxy/cpuset-t.h
@@ -9,6 +9,7 @@
 #ifdef __FreeBSD__
 #include <sys/_cpuset.h>
 #include <sys/cpuset.h>
+#include <strings.h>
 #endif
 #endif