nvme: fix compilation error

nvme: fix compilation error

gcc 7.4.0.

nvme.c: In function ‘print_relatives’:
nvme.c:3505:3: error: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Werror=unused-result]
   asprintf(&path, "/sys/class/nvme/%s", devicename);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
nvme.c:3509:3: error: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Werror=unused-result]
   asprintf(&path, "/sys/block/%s/device", devicename);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
Makefile:57: recipe for target 'nvme' failed
make: *** [nvme] Error 1

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>