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>
1 file changed