LinuxKPI: avoid -Werror=unused-value in sort() from BUILD_BUG_ON_ZERO()

The BUILD_BUG_ON_ZERO() macro returns an (int)0 if it does not fail
at build time. LinuxKPI sort() has it as a guard for an unsupported
argument but ignores the return value.

This leads to gcc complaining:

/usr/src/sys/compat/linuxkpi/common/include/linux/build_bug.h:60:33: error: statement with no effect [-Werror=unused-value]
   60 | #define BUILD_BUG_ON_ZERO(x)    ((int)sizeof(struct { int:-((x) != 0); }))
      |                                 ^
/usr/src/sys/compat/linuxkpi/common/include/linux/sort.h:37:9: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
   37 |         BUILD_BUG_ON_ZERO(swap);                        \
      |         ^~~~~~~~~~~~~~~~~
/usr/src/sys/contrib/dev/rtw89/core.c:2575:9: note: in expansion of macro 'sort'
 2575 |         sort(drift, RTW89_BCN_TRACK_STAT_NR, sizeof(*drift), cmp_u16, NULL);

Change to BUILD_BUG_ON() for the statement version.

Reported by:	CI
Co-authored-by:	bz
Approved by:	emaste (mentor)
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D55634

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

@@ -34,7 +34,7 @@
 #include <sys/libkern.h>
 
 #define	sort(base, num, size, cmp, swap)	do {	\
-	BUILD_BUG_ON_ZERO(swap);			\
+	BUILD_BUG_ON(swap);				\
 	qsort(base, num, size, cmp);			\
 } while (0)