trace: fix build with gcc 10

trace: fix build with gcc 10

Prevent from writing beyond the allocated memory.

GCC 10 compiling output:
eal_common_trace_utils.c: In function 'eal_trace_dir_args_save':
eal_common_trace_utils.c:290:24: error: '__builtin___sprintf_chk'   \
	may write a terminating nul past the end of the destination \
	[-Werror=format-overflow=]
  290 |  sprintf(dir_path, "%s/", optarg);
      |                        ^

Fixes: 8af866df8d8c ("trace: add trace directory configuration parameter")

Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Tested-by: Lijian Zhang <lijian.zhang@arm.com>
Acked-by: Sunil Kumar Kori <skori@marvell.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>