test/blobfs: rocksdb building error with gcc9

There's a small error when building rocksdb in blobfs db_bench
with gcc9:
"strncpy output truncated before terminating nul copying as many
bytes from a string as its length [-Werror=stringop-truncation]"

It's not a real bug for in the code "strncpy" truncates the null
terminator but set it later, which has been reported here:
https://github.com/facebook/rocksdb/issues/4017


Just adding "-Wno-error=stringop-truncation" to silent the compiler.

Signed-off-by: Richael Zhuang <richael.zhuang@arm.com>
Change-Id: I95993605f0376f5c101fb487596721b384319bcb
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1594


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>