fix building with 2.6.18+ headers

http://bugs.gentoo.org/150907

--- strace-4.5.14/system.c
+++ strace-4.5.14/system.c
@@ -1591,6 +1591,10 @@
 #endif
 
 #ifdef LINUX
+/* linux-2.6.18+ has dropped this enum */
+#ifndef CTL_PROC
+# define CTL_PROC 4
+#endif
 static const struct xlat sysctl_root[] = {
 	{ CTL_KERN, "CTL_KERN" },
 	{ CTL_VM, "CTL_VM" },