mesa: fix highly parallel builds

Somehow i was able to reproduce it very reliably on ppc-musl,
while barely at all anywhere else.

srcpkgs/mesa/patches/parallel-build.patch

@@ -0,0 +1,30 @@
+From 158c45a9a2555afca5adcfcebfa8e2dd3ff75507 Mon Sep 17 00:00:00 2001
+From: q66 <daniel@octaforge.org>
+Date: Sat, 7 Mar 2020 05:00:24 +0100
+Subject: [PATCH] depend on sha1_h in src/vulkan/overlay-layer
+
+Without this, the dependency ordering gets messed up and during
+highly parallel builds the git_sha1.h file gets generated later
+than overlay-layer is compiled. That results in build errors like:
+
+overlay.cpp:31:10: fatal error: git_sha1.h: No such file or directory
+---
+ src/vulkan/overlay-layer/meson.build | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git src/vulkan/overlay-layer/meson.build src/vulkan/overlay-layer/meson.build
+index 5b553d0..51e053e 100644
+--- src/vulkan/overlay-layer/meson.build
++++ src/vulkan/overlay-layer/meson.build
+@@ -38,7 +38,7 @@ vklayer_files = files(
+ 
+ vklayer_mesa_overlay = shared_library(
+   'VkLayer_MESA_overlay',
+-  vklayer_files, overlay_spv,
++  [vklayer_files, sha1_h], overlay_spv,
+   c_args : [c_vis_args, no_override_init_args, vulkan_wsi_args],
+   cpp_args : [cpp_vis_args, vulkan_wsi_args],
+   dependencies : [idep_vulkan_util, idep_mesautil, vulkan_wsi_deps, libimgui_core_dep, dep_dl],
+-- 
+2.25.1
+