build: Silence libtoolize warnings

When running the autogen.sh script, libtoolize complains thusly:

	libtoolize: Consider adding `AC_CONFIG_MACRO_DIR([m4])' to configure.ac and
	libtoolize: rerunning libtoolize, to keep the correct libtool macros in-tree.
	libtoolize: Consider adding `-I m4' to ACLOCAL_AMFLAGS in Makefile.am.

Silence the warnings by following libtoolize's advice.

Signed-off-by: Thierry Reding <treding@nvidia.com>
diff --git a/Makefile.am b/Makefile.am
index 9fc04ee..191dcc9 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,3 +1,5 @@
+ACLOCAL_AMFLAGS = -I m4
+
 bin_PROGRAMS =
 noinst_PROGRAMS =
 libexec_PROGRAMS =