make: avo_alloca.h works around a very old Solaris bug, delete it

@@ -30,11 +30,11 @@
  */
 
 /*
  * Included files
  */
-#include <avo/avo_alloca.h>             /* alloca() */
+#include <alloca.h>             /* alloca() */
 #include <errno.h>              /* errno */
 #include <fcntl.h>              /* fcntl() */
 #include <mk/defs.h>
 #include <mksh/macro.h>         /* expand_value(), expand_macro() */
 #include <mksh/misc.h>          /* getmem() */