dpdk: fix compiling issue with clang

dpdk: fix compiling issue with clang

When building vpp image with below command, clang reports warnings/errors
on the unused option '-L', as it's not linking stage, and does not require
linking libraries.

In dpdk.mk, the linking library path should be attached to
DPDK_EXTRA_LDFLAGS, instead of DPDK_EXTRA_CFLAGS

$ make build-release CC=clang V=1

clang -Wp,-MD,./.ark_ddm.o.d.tmp  -fPIE -fPIC -pthread -I/root/origin/build-root/build-vpp-native/external/dpdk-20.02/lib/librte_eal/linux/eal/include  -march=armv8-a+crc -DRTE_MACHINE_CPUFLAG_NEON -DRTE_MACHINE_CPUFLAG_CRC32  -I/root/origin/build-root/build-vpp-native/external/dpdk-20.02/arm64-armv8a-linuxapp-clang/include -DRTE_USE_FUNCTION_VERSIONING -include /root/origin/build-root/build-vpp-native/external/dpdk-20.02/arm64-armv8a-linuxapp-clang/include/rte_config.h
-D_GNU_SOURCE -O3 -I./ -W -Wall -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Wold-style-definition -Wpointer-arith -Wnested-externs -Wcast-qual -Wformat-nonliteral -Wformat-security -Wundef -Wwrite-strings -Wdeprecated -Wno-missing-field-initializers -Wno-address-of-packed-member -Werror   -g -mtune=generic -L/root/origin/build-root/install-vpp-native/external/lib -I/root/origin/build-root/install-vpp-native/external/include -o ark_ddm.o -c
/root/origin/build-root/build-vpp-native/external/dpdk-20.02/drivers/net/ark/ark_ddm.c
== Build drivers/vdpa/ifc
clang: warning: argument unused during compilation: '-L/root/origin/build-root/install-vpp-native/external/lib' [-Wunused-command-line-argument]
clang: error: argument unused during compilation: '-L/root/origin/build-root/install-vpp-native/external/lib' [-Werror,-Wunused-command-line-argument]

Type: fix

Change-Id: If8fd9b19d0aff9d3c27d77e78cd3064bb1ad6565
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Jieqiang Wang <Jieqiang.Wang@arm.com>
Reviewed-by: Govindarajan Mohandoss <Govindarajan.Mohandoss@arm.com>
(cherry picked from commit 88d6ce2bee53b7c9f876aa78c5db8ab1ea75c08a)