kern_racct.c: Don't compile if RACCT undefined

Just skip compiling this file if RACCT isn't defined.  This allows to
skip including headers that no code uses at all, and also to remove the
whole file's #ifdef/#endif bracketing.

Reviewed by:    markj
MFC after:      2 weeks
Sponsored by:   The FreeBSD Foundation

sys/conf/files

@@ -3795,7 +3795,7 @@ kern/kern_priv.c		standard
 kern/kern_proc.c		standard
 kern/kern_procctl.c		standard
 kern/kern_prot.c		standard
-kern/kern_racct.c		standard
+kern/kern_racct.c		optional racct
 kern/kern_rangelock.c		standard
 kern/kern_rctl.c		standard
 kern/kern_resource.c		standard

sys/kern/kern_racct.c

@@ -59,8 +59,6 @@
 #include <sys/rctl.h>
 #endif
 
-#ifdef RACCT
-
 FEATURE(racct, "Resource Accounting");
 
 /*
@@ -1364,5 +1362,3 @@ racct_init(void)
 	prison0.pr_prison_racct = prison_racct_find("0");
 }
 SYSINIT(racct, SI_SUB_RACCT, SI_ORDER_FIRST, racct_init, NULL);
-
-#endif /* !RACCT */