[PATCH Storable 2.06] const declaration fixup

With recent versions of Compaq C under OpenVMS, the latest Storable 
version fails to compile like so:

CC/DECC 
/Include=[]/Standard=Relaxed_ANSI/Prefix=All/Obj=.obj/NoList/NOANSI_ALIAS/Define=("VERSION=""2.06""","XS_VERSION=""2.06""")/
Include=(perl_root:[lib.VMS_AXP.5_6_1.CORE])/Optimize  STORABLE.c

     WRITE(header, length);
....^
%CC-W-NOTCONSTQUAL, In this statement, the referenced type of the 
pointer value "header" is const, but the referenced type of the target 
of this assignment is not.
at line number 3369 in file D0:[CRAIG.STORABLE-2_06]STORABLE.XS;1

The patch below takes care of this, after which the extension builds ok 
  and all tests pass under Perl 5.6.1, OpenVMS Alpha 7.3-1, Compaq C 6.5.

• [PATCH Storable 2.06] const declaration fixup by Craig A. Berry
• Re: [PATCH Storable 2.06] const declaration fixup by Nick Ing-Simmons
• Re: [PATCH Storable 2.06] const declaration fixup by sthoenna
• Re: [PATCH Storable 2.06] const declaration fixup by Craig A. Berry
• Re: [PATCH Storable 2.06] const declaration fixup by Abhijit Menon-Sen