Fix chrome build: Use spaces (not tabs) in .gn files

Build error: `You got a tab character in here. Tabs are evil. Convert to spaces.`

Bug: b/139528538
Change-Id: I937f96d3aeae48103510c0faab6bec9499a290dc
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38588
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
diff --git a/src/Vulkan/BUILD.gn b/src/Vulkan/BUILD.gn
index 94f067f..0efb0a7 100644
--- a/src/Vulkan/BUILD.gn
+++ b/src/Vulkan/BUILD.gn
@@ -136,7 +136,7 @@
     "VkSampler.cpp",
     "VkSemaphore.cpp",
     "VkShaderModule.cpp",
-	"VkStringify.cpp",
+    "VkStringify.cpp",
     "Vulkan.rc",
     "libVulkan.cpp",
     "main.cpp",