configure: Add AM_PROG_AR to silence automake warning

/usr/share/automake-1.13/am/ltlibrary.am: warning: 'ct_helper_tns.la': linking libtool libraries using a non-POSIX /usr/share/automake-1.13/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
src/helpers/Makefile.am:3: while processing Libtool library 'ct_helper_tns.la'

---
 configure.ac | 1 +
 1 file changed, 1 insertion(+)

index f0800d6..523a192 100644
@@ -14,6 +14,7 @@ AC_SUBST([libdl_LIBS])
 
 AC_PROG_CC
 AC_DISABLE_STATIC
+AM_PROG_AR
 AM_PROG_LIBTOOL
 AC_PROG_INSTALL
 AC_PROG_LN_S

2.3.6

Series for conntrack-tools applied, thanks.