bpf: also include toolchain standard header as system header

bpf: also include toolchain standard header as system header

Also include toolchain standard header as system header. These are
required by xdp-tools that try to include stddef.h and stdbool.h for
some tools. These header are usually in /lib/gcc/../include but musl
also have some special variant in /include.

To fix compilation of xdp-tools, also include these standard header.
These header should follow ISO C standard and should not introduce
regression in bpf tools making them specific to an arch.

Link: https://github.com/openwrt/openwrt/pull/15390
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>