Fix windows compilation warnings

Fix some warnings in the async code when compiling on windows.

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/async/arch/async_posix.h

@@ -99,9 +99,5 @@ static inline int ASYNC_FIBRE_swapcontext(ASYNC_FIBRE *o, ASYNC_FIBRE *n, int r)
 int ASYNC_FIBRE_init(ASYNC_FIBRE *fibre);
 void ASYNC_FIBRE_free(ASYNC_FIBRE *fibre);
 
-int async_pipe(int *pipefds);
-int async_write1(int fd, const void *buf);
-int async_read1(int fd, void *buf);
-
 # endif
 #endif