gcc 7 warning

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

Bemused by:

xenpmd.c:90:36: error: '%s' directive output may be truncated writing up to 511 bytes into a region of size 271 [-Werror=format-truncation=]
     #define BATTERY_INFO_FILE_PATH "/tmp/battery/%s/info"
                                    ^
xenpmd.c:113:52: note: in expansion of macro 'BATTERY_INFO_FILE_PATH'
             snprintf(file_name, sizeof(file_name), BATTERY_INFO_FILE_PATH,
                                                    ^~~~~~~~~~~~~~~~~~~~~~
xenpmd.c:113:13: note: '__builtin_snprintf' output between 19 and 530 bytes into a destination of size 284
             snprintf(file_name, sizeof(file_name), BATTERY_INFO_FILE_PATH,
             ^

while trying to build xentools411. I thought that was the point of
snprintf...

Cheers,

Patrick