Update SPIRV-Headers

The SPIRV-Headers version for vulkan-cts-1.2.6 was ahead of 1.2.7. This
change brings SPIRV-Headers in 1.2.7 up to the same version.

Components: Vulkan
Change-Id: Ifff93c5251183d3a6ee3ef3f508e88274332116f
diff --git a/external/fetch_sources.py b/external/fetch_sources.py
index 350daf9..35ff926 100644
--- a/external/fetch_sources.py
+++ b/external/fetch_sources.py
@@ -328,7 +328,7 @@
 	GitRepo(
 		"https://github.com/KhronosGroup/SPIRV-Headers.git",
 		None,
-		"cf653e4ca4858583802b0d1656bc934edff6bd7f",
+		"449bc986ba6f4c5e10e32828783f9daef2a77644",
 		"spirv-headers"),
 	GitRepo(
 		"https://github.com/google/amber.git",