package/multipath-tools: disable -Werror

package/multipath-tools: disable -Werror

Set the new WARNFLAGS to "" which has been added since version 0.8.5 and
https://github.com/opensvc/multipath-tools/commit/82f1b164cb21c9632b3c73f865d97777c7a61e0d

Otherwise, -Werror will raise the following build failure:

/srv/storage/autobuild/run/instance-3/output-1/host/bin/mipsel-linux-gcc --std=gnu99 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O2  -D_FORTIFY_SOURCE=1  -Werror -Wall -Wextra -Wformat=2 -Werror=implicit-int -Werror=implicit-function-declaration -Werror=format-security -Wno-clobbered -Wno-error=clobbered -Werror=cast-qual -Werror=discarded-qualifiers -pipe -DBIN_DIR=\"/sbin\" -DLIB_STRING=\"lib\" -DRUN_DIR=\"run\" -MMD -MP -fPIC -I.. -I../../libmultipath/nvme -Wp,-D_FORTIFY_SOURCE=2  -c -o nvme.o nvme.c
<command-line>: error: "_FORTIFY_SOURCE" redefined [-Werror]

Fixes:
 - http://autobuild.buildroot.org/results/71f7661e7d26ca8608e902eee9f2a92376b00601

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>