Stop warnings in gcc where "a" is const passed as a non-const argument.

crypto/bio/bio.h

@@ -411,7 +411,7 @@ typedef int asn1_ps_func(BIO *b, unsigned char **pbuf, int *plen, void *parg);
 #define BIO_set_accept_port(b,name) BIO_ctrl(b,BIO_C_SET_ACCEPT,0,(char *)name)
 #define BIO_get_accept_port(b)	BIO_ptr_ctrl(b,BIO_C_GET_ACCEPT,0)
 /* #define BIO_set_nbio(b,n)	BIO_ctrl(b,BIO_C_SET_NBIO,(n),NULL) */
-#define BIO_set_nbio_accept(b,n) BIO_ctrl(b,BIO_C_SET_ACCEPT,1,(n)?"a":NULL)
+#define BIO_set_nbio_accept(b,n) BIO_ctrl(b,BIO_C_SET_ACCEPT,1,(n)?(void *)"a":NULL)
 #define BIO_set_accept_bios(b,bio) BIO_ctrl(b,BIO_C_SET_ACCEPT,2,(char *)bio)
 
 #define BIO_BIND_NORMAL			0