android: vulkan/util: fix generating vk_enum_to_str.*

android: vulkan/util: fix generating vk_enum_to_str.*

The gen_enum_to_str.py generates vk_enum_to_str.c and its header at once.
However, the makefiles incorrectly list both files parallel with the same
recipes. That means both two files may be generated simultaneously by two
processes. The generating files may be truncated by another process, as
shown below:

$ cd $OUT/obj/STATIC_LIBRARIES/libmesa_vulkan_util_intermediates/util
$ ls -l

-rw-rw-r-- 1 lh lh 193713 Jul  5 13:31 vk_enum_to_str.c
-rw-rw-r-- 1 lh lh   4609 Jul  5 13:31 vk_enum_to_str.d
-rw-rw-r-- 1 lh lh      0 Jul  5 16:21 vk_enum_to_str.h

Let one file depends on the other with empty recipe to avoid the issue.

Signed-off-by: Chih-Wei Huang <cwhuang@linux.org.tw>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>