Makefile.tinydtls: fixup dtls_debug.c path

Change-Id: Ie388ee5d86a32abfd102e4783e6b921b0fc98cd0
Signed-off-by: Gaëtan Harter <gaetan.harter@inria.fr>
diff --git a/Makefile.tinydtls b/Makefile.tinydtls
index 88affc4..ee7b6de 100644
--- a/Makefile.tinydtls
+++ b/Makefile.tinydtls
@@ -32,5 +32,5 @@
 
 # This activates debugging support
 # CFLAGS += -DNDEBUG
-tinydtls_src += debug.c
+tinydtls_src += dtls_debug.c