Makefile.am: fix parallel issue

There might be no src dir if src/builtin.h runs earlier, create it to
fix the race issue:

src/genbuiltin nfctype1 nfctype2 nfctype3 nfctype4 p2p > src/builtin.h
/bin/sh: src/builtin.h: No such file or directory

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>