Update .gitignore for zlib.h

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
diff --git a/tools/.gitignore b/tools/.gitignore
index 979f2da..df3500d 100644
--- a/tools/.gitignore
+++ b/tools/.gitignore
@@ -15,3 +15,4 @@
 /fdt_strerror.c
 /fdt_wip.c
 /libfdt_internal.h
+/zlib.h