this commit fixes musl build

precompiled headers are not strictly necessary according to
https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html

precompiled headers will probably be broken anyway in cross-compile cases

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>

Signed-off-by: lijpsummer <summer@wrtnode.com>

Signed-off-by: Jianping Li <summer@wrtnode.com>

Signed-off-by: Nicolas Thill <nico@openwrt.org>

[Etienne CHAMPETIER: compile tested for ar71xx]
Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>
Signed-off-by: Jialei Hao <schumy@wrtnode.com>