fix build regression in sysconf for archs with variable page size

commit 5ce3737931bb411a8d167356d4d0287b53b0cbdc removed the inclusion
of libc.h from this file as spurious, but it's needed to get PAGE_SIZE
on archs where PAGE_SIZE is not a constant defined by limits.h.

src/conf/sysconf.c

@@ -5,6 +5,7 @@
 #include <signal.h>
 #include <sys/sysinfo.h>
 #include "syscall.h"
+#include "libc.h"
 
 #define JT(x) (-256|(x))
 #define VER JT(1)