build: add AM_PROG_AR to configure_ac

Add missing AM_PROG_AR to configure_ac

Change-Id: I260c1b1b4f22989700f5dae5f5e49d22686c06c9
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index b2f823e..446df9c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -9,6 +9,7 @@
 AC_SUBST(VERSION)
 
 # Checks for programs
+AM_PROG_AR
 AC_PROG_CC
 AC_PROG_INSTALL
 AC_PROG_MAKE_SET