Fix gcc build by removing redeclaration

Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D20622

stand/efi/boot1/boot_module.h

@@ -109,5 +109,4 @@ extern const boot_module_t zfs_module;
 
 /* Functions available to modules. */
 extern void add_device(dev_info_t **devinfop, dev_info_t *devinfo);
-extern int vsnprintf(char *str, size_t sz, const char *fmt, va_list ap);
 #endif