libfstools: Check return values for fread and system

libfstools: Check return values for fread and system

libfstools/rootdisk.c: In function 'rootdisk_volume_identify':
libfstools/rootdisk.c:172:7: error: ignoring return value of 'fread', declared with attribute warn_unused_result [-Werror=unused-result]
  fread(&magic, sizeof(magic), 1, f);
       ^
libfstools/rootdisk.c:179:7: error: ignoring return value of 'fread', declared with attribute warn_unused_result [-Werror=unused-result]
  fread(&magic, sizeof(magic), 1, f);
       ^
libfstools/rootdisk.c: In function 'rootdisk_volume_init':
libfstools/rootdisk.c:268:9: error: ignoring return value of 'system', declared with attribute warn_unused_result [-Werror=unused-result]
   system(str);
         ^
cc1: all warnings being treated as errors

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>