haproxy: Pass down TARGET_CPPFLAGS for lua build

haproxy: Pass down TARGET_CPPFLAGS for lua build

Some external toolchaind don't automatically search for headers in
$(STAGING_DIR), make sure we do pass TARGET_CPPFLAGS as appropriate.
This fixes build errors such as:

mipsel-linux-gnu-gcc -O2 -Wall -Wextra -DLUA_COMPAT_5_2 -DLUA_USE_LINUX
-c -o lua.o lua.c
lua.c:80:31: fatal error: readline/readline.h: No such file or directory
 #include <readline/readline.h>
                               ^
compilation terminated.
<builtin>: recipe for target 'lua.o' failed
make[6]: *** [lua.o] Error 1

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>