zlib: fix compilation failures with Sun C Compilaer

Do this by removing a couple of useless return statements.  Without this
change, compilation with Sun C Compiler 5.9 (SunOS_i386 Patch 124868-15
2010/08/11) fails with the following message:

  "zlib.c", line 192: void function cannot return value
  "zlib.c", line 201: void function cannot return value
  cc: acomp failed for zlib.c

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

zlib.c

@@ -189,7 +189,7 @@ void git_deflate_init_gzip(git_zstream *strm, int level)
 	 * Use default 15 bits, +16 is to generate gzip header/trailer
 	 * instead of the zlib wrapper.
 	 */
-	return do_git_deflate_init(strm, level, 15 + 16);
+	do_git_deflate_init(strm, level, 15 + 16);
 }
 
 void git_deflate_init_raw(git_zstream *strm, int level)
@@ -198,7 +198,7 @@ void git_deflate_init_raw(git_zstream *strm, int level)
 	 * Use default 15 bits, negate the value to get raw compressed
 	 * data without zlib header and trailer.
 	 */
-	return do_git_deflate_init(strm, level, -15);
+	do_git_deflate_init(strm, level, -15);
 }
 
 int git_deflate_abort(git_zstream *strm)