Update gm_file.h

Modification done with  ../lib/file.c

"read() buffer overflow on file /proc/stat"

At the very first read :
- the file is read in a buffer sized with 4096 bytes
- if 4096 is not enough then the buffer i reallocated until the size is sufficient.
- then the right size is memorized in buffersize
Then all other are read with buffersize size with no possible re-allocation.

This behaviour is an issue with files like /proc/stat which can grow with time leading to infinite "read() buffer overflow on file .."

The purpose of the modification is to let the reallocation always possible