Update glslang tag

Update glslang tag to vulkan-1.1-rc4

Affects: dEQP-VK.*

Change-Id: Idd20ba8a7c3d58124062da119af5186acc08a4d2
diff --git a/external/fetch_sources.py b/external/fetch_sources.py
index fe40b8b..e8840a5 100644
--- a/external/fetch_sources.py
+++ b/external/fetch_sources.py
@@ -257,7 +257,7 @@
 	GitRepo(
 		"https://gitlab.khronos.org/GLSL/glslang.git",
 		"git@gitlab.khronos.org:GLSL/glslang.git",
-		"vulkan-1.1-rc1",
+		"vulkan-1.1-rc4",
 		"glslang"),
 	GitRepo(
 		"https://gitlab.khronos.org/spirv/SPIRV-Headers.git",