libwebsockets

Fix build with musl libc

Fix building libwebsockets with the musl C libary.

<sys/cdefs.h> is an internal glibc header and should be avoided in user code.

__P() was used for compatibility with some old K&R C compilers, when there were
no prototypes (which were introduced to C with C89). As supporting legacy
non-ANSI compilers is nowadays not necessary anymore get rid of the unnecessary
function prototype using __P().