Don't declare __assfail as static

It gets called by dmu_buf_init_user, which is inline but not static.  So it
needs global linkage itself.

Reported by:	GCC-6
MFC after:	17 days
X-MFC-With:	329722

cddl/contrib/opensolaris/lib/libzpool/common/kernel.c

@@ -32,6 +32,7 @@
 #include <string.h>
 #include <zlib.h>
 #include <libgen.h>
+#include <sys/assfail.h>
 #include <sys/spa.h>
 #include <sys/stat.h>
 #include <sys/processor.h>

sys/cddl/compat/opensolaris/sys/assfail.h

@@ -48,7 +48,9 @@ void assfail3(const char *, uintmax_t, const char *, uintmax_t, const char *,
 #ifndef HAVE_ASSFAIL
 extern int aok;
 
-static __inline int
+__inline int __assfail(const char *expr, const char *file, int line);
+
+__inline int
 __assfail(const char *expr, const char *file, int line)
 {