git-compat-util.h: avoid using c99 flex array feature with Sun compiler 5.8

git-compat-util.h: avoid using c99 flex array feature with Sun compiler 5.8

The Sun c99 compiler as recent as version 5.8 Patch 121016-06 2007/08/01
produces an error when compiling diff-delta.c.  This source file #includes
the delta.h header file which pre-declares a struct which is later defined
to contain a flex array member.  The Sun c99 compiler fails to compile
diff-delta.c and gives the following error:

  "diff-delta.c", line 314: identifier redeclared: create_delta
          current : function(pointer to const struct delta_index {unsigned long memsize, pointer to const void src_buf, unsigned long src_size, unsigned int hash_mask, array[-1] of pointer to struct index_entry {..} hash}, pointer to const void, unsigned long, pointer to unsigned long, unsigned long) returning pointer to void
          previous: function(pointer to const struct delta_index {unsigned long memsize, pointer to const void src_buf, unsigned long src_size, unsigned int hash_mask, array[-1] of pointer to struct index_entry {..} hash}, pointer to const void, unsigned long, pointer to unsigned long, unsigned long) returning pointer to void : "delta.h", line 44
  c99: acomp failed for diff-delta.c

So, avoid using this c99 feature when compiling with the Sun c compilers
version 5.8 and older (the most recent version tested).

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>