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

*** 30,40 ****
   */
  
  /*
   * Included files
   */
! #include <avo/avo_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() */
--- 30,40 ----
   */
  
  /*
   * Included files
   */
! #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() */