Fix Windows build

- use TINYCBOR_HEADERS in Makefile.nmake instead of using src

Author: vrahane

Makefile.nmake

@@ -43,5 +43,5 @@ tag:
 	@perl maketag.pl
 
 {src\}.c{src\}.obj:
-	$(CC) -nologo $(CFLAGS) -Isrc -DTINYCBOR_VERSION_SUFFIX="" -c -Fo$@ $<
+	$(CC) -nologo $(CFLAGS) -I$(TINYCBOR_HEADERS) -DTINYCBOR_VERSION_SUFFIX="" -c -Fo$@ $<