ynl-gen-cpp: add support for generating ovs_flow

Summary:
Enable ovs_flow code generation and fix compile failure
Backport from net-next record information about recursive struct.
Forward declarations if child struct in nested struct is define after it.
Recursive types are stored as vector not option, update attr_put and _attr_get to handle it correctly

net-next:
tools: ynl-gen: record information about recursive nests(commit 38329fcfb757b8215c07a77b6657721cc7e9530e)


Test Plan:
make sure code could be generated and compiled

Signed-off-by: Sheldon Wu <xzw2048@gmail.com>

Author: sheldnwu

generated/Makefile

@@ -17,7 +17,7 @@ GENS_PATHS=$(shell grep -nrI --files-without-match \
 		'protocol: netlink' ../Documentation/netlink/specs/)
 GENS_PATHS+=$(wildcard ../Documentation/netlink/specs/rt-*.yaml)
 GENS_ALL=$(patsubst ../Documentation/netlink/specs/%.yaml,%,${GENS_PATHS})
-GENS=$(filter-out ovs_flow nlctrl,${GENS_ALL})
+GENS=$(filter-out nlctrl,${GENS_ALL})
 SRCS=$(patsubst %,%-user.cpp,${GENS})
 HDRS=$(patsubst %,%-user.hpp,${GENS})
 OBJS=$(patsubst %,%-user.cpp.o,${GENS})