selftests/bpf: add -std=gnu11 to BPF_CFLAGS and CFLAGS

selftests/bpf: add -std=gnu11 to BPF_CFLAGS and CFLAGS

Latest versions of GCC BPF use C23 standard by default. This causes
compilation errors in vmlinux.h due to bool types declarations.

Add -std=gnu11 to BPF_CFLAGS and CFLAGS. This aligns with the version
of the standard used when building the kernel currently [1].

For more details see the discussions at [2] and [3].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile#n465
[2] https://lore.kernel.org/bpf/EYcXjcKDCJY7Yb0GGtAAb7nLKPEvrgWdvWpuNzXm2qi6rYMZDixKv5KwfVVMBq17V55xyC-A1wIjrqG3aw-Imqudo9q9X7D7nLU2gWgbN0w=@pm.me/
[3] https://lore.kernel.org/bpf/20250106202715.1232864-1-ihor.solodrai@pm.me/

CC: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Link: https://lore.kernel.org/r/20250107235813.2964472-1-ihor.solodrai@pm.me
Signed-off-by: Alexei Starovoitov <ast@kernel.org>