Remove export of GLVersion from GLAD

This symbol conflicts with a duplicate symbol when we link Dawn into
Chromium on Linux.

Bug: chromium:877147
Change-Id: Iadf8ac81760085a04d82ea9116e367a1bd17cd4d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/5260
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
diff --git a/third_party/glad/include/glad/glad.h b/third_party/glad/include/glad/glad.h
index 46e8798..2825675 100644
--- a/third_party/glad/include/glad/glad.h
+++ b/third_party/glad/include/glad/glad.h
@@ -7,7 +7,7 @@
     APIs: gl=4.5
     Profile: core
     Extensions:
-        
+
     Loader: True
     Local files: False
     Omit khrplatform: False
@@ -81,7 +81,7 @@
 # endif
 #endif
 
-GLAPI struct gladGLversionStruct GLVersion;
+extern struct gladGLversionStruct GLVersion;
 
 GLAPI int gladLoadGL(void);