build: Fix the building of tests as well
In file included from ../../include/osmocom/core/msgb.h:25:0,
from ../../tests/sms/sms_test.c:31:
../../include/osmocom/core/bits.h:6:35: fatal error: osmocom/core/bit16gen.h: No such file or directory
#include <osmocom/core/bit16gen.h>
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 8db2533..b0701f8 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,4 +1,4 @@
-AM_CPPFLAGS = -I$(top_srcdir)/include
+AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_builddir)/include
AM_CFLAGS = -Wall
check_PROGRAMS = timer/timer_test sms/sms_test ussd/ussd_test \