libotr: Fix QA-issue where also files from buildhost would be included

Warning was:
cc1: warning: include location "/usr/include" is unsafe for
cross-compilation [-Wpoison-system-directories]

Upstream-Status: Pending

Signed-off-by: Jakob Drexel <jake42@rommel.stw.uni-erlangen.de>

--- a/toolkit/Makefile.am	2012-05-03 15:52:22.000000000 +0200
+++ b/toolkit/Makefile.am	2014-01-13 12:12:26.030369357 +0100
@@ -1,4 +1,4 @@
-AM_CPPFLAGS = -I$(includedir) -I../src @LIBGCRYPT_CFLAGS@
+AM_CPPFLAGS = -I../src @LIBGCRYPT_CFLAGS@
 
 noinst_HEADERS = aes.h ctrmode.h parse.h sesskeys.h readotr.h sha1hmac.h