Build failure: non-SMP after SVN r324778

[Michael Butler]

If SMP is not defined, as it isn't on my last remaining i386 platform, 
the build fails with:

Building /usr/obj/usr/src/sys/SARAH/kern_mutex.o
--- kern_mutex.o ---
/usr/src/sys/kern/kern_mutex.c:313:3: error: implicit declaration of 
function '_mtx_lock_spin' is invalid in C99 
[-Werror,-Wimplicit-function-declaration]
                 _mtx_lock_spin(m, v, opts, file, line);
                 ^
/usr/src/sys/kern/kern_mutex.c:313:3: error: this function declaration 
is not a prototype [-Werror,-Wstrict-prototypes]
2 errors generated.
*** [kern_mutex.o] Error code 1

make[2]: stopped in /usr/obj/usr/src/sys/SARAH

	imb