didn't compie dtrace without INVARIANTS.

Hi jb!

	I couldn't compile dtrace modules by following causes.
	I found why SX_UNLOCKED is not defined.  In sys/sx.h,
	SX_UNLOCKED is defined like following.
#if defined(INVARIANTS) || defined(INVARIANT_SUPPORT)
  ;
#define SX_UNLOCKED		LA_UNLOCKED
  :
#endif

	Would you please fix this issue?

===> dtrace/dtrace (all)
cc -O2 -pipe -march=pentium-m -DDIS_MEM -DSMP -DDEBUG -fno-strict-aliasing -Werror -D_KERNEL -DKLD_MODULE -std=c99 -nostdinc  -I/usr/src/sys/modules/dtrace/dtrace/../../../cddl/compat/opensolaris -I/usr/src/sys/modules/dtrace/dtrace/../../../cddl/dev/dtrace -I/usr/src/sys/modules/dtrace/dtrace/../../../cddl/dev/dtrace/i386 -I/usr/src/sys/modules/dtrace/dtrace/../../../cddl/contrib/opensolaris/uts/common -I/usr/src/sys/modules/dtrace/dtrace/../../.. -DHAVE_KERNEL_OPTION_HEADERS -include /usr/obj/usr/src/sys/MELFINA/opt_global.h -I. -I_at_ -I_at_/contrib/altq -finline-limit=8000 --param inline-unit-growth=100 --param large-function-growth=1000 -fno-common -g3 -I/usr/obj/usr/src/sys/MELFINA -mno-align-long-strings -mpreferred-stack-boundary=2  -mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3 -ffreestanding -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes  -Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual  -Wundef -Wno-pointer-sign -fformat-extensions -Wno-unknown-!
 pragmas -c /usr/src/sys/modules/dtrace/dtrace/../../../cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c
/usr/src/sys/modules/dtrace/dtrace/../../../cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c: In function 'dtrace_dif_subr':
/usr/src/sys/modules/dtrace/dtrace/../../../cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c:3283: error: 'SX_UNLOCKED' undeclared (first use in this function)
/usr/src/sys/modules/dtrace/dtrace/../../../cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c:3283: error: (Each undeclared identifier is reported only once
/usr/src/sys/modules/dtrace/dtrace/../../../cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c:3283: error: for each function it appears in.)