selftests/bpf: Fix conflicts with built-in functions in bench_local_storage_create

The fork function in gcc is considered a built in function due to
being used by libgcov when building with gnu extensions.

Rename fork to sched_process_fork to prevent this conflict.

See details:
https://github.com/gcc-mirror/gcc/commit/d1c38823924506d389ca58d02926ace21bdf82fa
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82457



Fixes the following error:

In file included from progs/bench_local_storage_create.c:6:
progs/bench_local_storage_create.c:43:14: error: conflicting types for
built-in function 'fork'; expected 'int(void)'
[-Werror=builtin-declaration-mismatch]
   43 | int BPF_PROG(fork, struct task_struct *parent, struct
task_struct *child)
      |              ^~~~

Fixes: cbe9d93d ("selftests/bpf: Add bench for task storage creation")
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230331075848.1642814-1-james.hilliard1@gmail.com