openbsd-compat/bsd-asprintf: add <stdio.h> include for vsnprintf

Fixes the following build failure with Clang 15 on musl:
```
bsd-asprintf.c:51:8: error: call to undeclared library function 'vsnprintf' with type 'int (char *, unsigned long, const char *, struct __va_list_tag *)'; ISO C99 and laterclang -O2 -pipe -fdiagnostics-color=always -frecord-gcc-switches -pipe -Wunknown-warning-option -Qunused-arguments -Wall -Wpointer-arith -Wuninitialized -Wsign-compare -Wformat-security -Wsizeof-pointer-memaccess -Wno-pointer-sign -Wno-unused-result -Wmisleading-indentation -Wbitwise-instead-of-logical -fno-strict-aliasing -mretpoline  -ftrapv -fzero-call-used-regs=all -fno-builtin-memset -fstack-protector-strong -fPIE   -I. -I.  -D_XOPEN_SOURCE=600 -D_BSD_SOURCE -D_DEFAULT_SOURCE -DSSHDIR=\"/etc/ssh\" -D_PATH_SSH_PROGRAM=\"/usr/bin/ssh\" -D_PATH_SSH_ASKPASS_DEFAULT=\"/usr/lib/misc/ssh-askpass\" -D_PATH_SFTP_SERVER=\"/usr/lib/misc/sftp-server\" -D_PATH_SSH_KEY_SIGN=\"/usr/lib/misc/ssh-keysign\" -D_PATH_SSH_PKCS11_HELPER=\"/usr/lib/misc/ssh-pkcs11-helper\" -D_PATH_SSH_SK_HELPER=\"/usr/lib/misc/ssh-sk-helper\" -D_PATH_SSH_PIDDIR=\"/run\" -D_PATH_PRIVSEP_CHROOT_DIR=\"/var/empty\" -DHAVE_CONFIG_H -c cipher-aes.c -o cipher-aes.o
 do not support
      implicit function declarations [-Wimplicit-function-declaration]
        ret = vsnprintf(string, INIT_SZ, fmt, ap2);
              ^
bsd-asprintf.c:51:8: note: include the header <stdio.h> or explicitly provide a declaration for 'vsnprintf'
1 error generated.
```

openbsd-compat/bsd-asprintf.c

@@ -32,6 +32,7 @@
 
 #include <errno.h>
 #include <stdarg.h>
+#include <stdio.h>
 #include <stdlib.h>
 
 #define INIT_SZ	128