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);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[rexim]

Compiled on both GCC and Clang with -Wall -Wextra -Wunused-result -O3. Couldn't reproduce.

[theNoof]

your 'fix' produces new warning for me:

In file included from test.c:2:
nob.h: In function ‘nob_read_entire_file’:
nob.h:2584:18: warning: unused variable ‘nread’ [-Wunused-variable]
 2584 |     size_t const nread = fread(sb->items + sb->count, m, 1, f);
      |                  ^~~~~

assigning it to a variable and then never using it is kinda bad and produces warnings for nearly everyone.

different idea:

...
    (void) fread(sb->items + sb->count, m, 1, f);
...