LinuxKPI: change DECLARE_FLEX_ARRAY()

DECLARE_FLEX_ARRAY can be used inside a structure.  On FreeBSD due to
-Wgnu-variable-sized-type-not-at-end this yields an error.  Use [0]
instead of [] to overcome this.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	hselasky
Differential Revision: https://reviews.freebsd.org/D34350

sys/compat/linuxkpi/common/include/linux/kernel.h

@@ -700,7 +700,7 @@ hex2bin(uint8_t *bindst, const char *hexsrc, size_t binlen)
 }
 
 #define	DECLARE_FLEX_ARRAY(_t, _n)					\
-    struct { struct { } __dummy_ ## _n; _t _n[]; }
+    struct { struct { } __dummy_ ## _n; _t _n[0]; }
 
 /*
  * Checking if an option is defined would be easy if we could do CPP inside CPP.