build: add -I m4 to Makefile for ACLOCAL_AMFLAGS

Per suggestion during configuration, add m4 directory to
ACLOCAL_AMFLAGS.

Change-Id: I0c7ae12e02e5ef25f41c8a3b31b5338504ff9463
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index 0970c26..ea43c24 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,3 +1,5 @@
+ACLOCAL_AMFLAGS = -I m4
+
 CLEANFILES = *~
 
 sysconf_DATA = key_file