Fix warning when building with -O3

[ranjeethmahankali]

nob.h: In function ‘nob_read_entire_file’:
nob.h:1676:5: warning: ignoring return value of ‘fread’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
 1676 |     fread(sb->items + sb->count, m, 1, f);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~