Fix compile with -D_std_malloc (re: f9c127e) (#130)

src/cmd/ksh93/include/jobs.h:
- The commit that removed legacy code mistakenly removed the
  definition of vmbusy() required for ksh to compile with
  -D_std_malloc. Ksh assumes vmbusy is always a macro, even
  when _std_malloc is defined. This commit reintroduces the
  _std_malloc definition of vmbusy to fix undefined
  reference errors.

src/cmd/ksh93/include/jobs.h

@@ -120,6 +120,8 @@ extern struct jobs job;
 #else
 #define vmbusy()	(vmstat(0,0)!=0)
 #endif
+#else
+#define vmbusy()	0
 #endif
 
 #define job_lock()	(job.in_critical++)