wrapper: give zlib wrappers their own translation unit

Programs using xmalloc() but not git_inflate() require -lz on the
linker command line because git_inflate() is in the same translation
unit as xmalloc().

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Makefile b/Makefile
index 1f1ce04..a8ba336 100644
--- a/Makefile
+++ b/Makefile
@@ -662,6 +662,7 @@
 LIB_OBJS += ws.o
 LIB_OBJS += wt-status.o
 LIB_OBJS += xdiff-interface.o
+LIB_OBJS += zlib.o
 
 BUILTIN_OBJS += builtin/add.o
 BUILTIN_OBJS += builtin/annotate.o