Fixed warnings with gcc option "-Wwrite-strings".

diff --git a/dlls/opengl32/opengl_ext.h b/dlls/opengl32/opengl_ext.h
index 32e81a2..eb8b84f 100644
--- a/dlls/opengl32/opengl_ext.h
+++ b/dlls/opengl32/opengl_ext.h
@@ -51,8 +51,8 @@
 
 
 typedef struct {
-  char  *name;     /* name of the extension */
-  char  *glx_name; /* name used on Unix's libGL */
+  const char  *name;     /* name of the extension */
+  const char  *glx_name; /* name used on Unix's libGL */
   void  *func;     /* pointer to the Wine function for this extension */
   void **func_ptr; /* where to store the value of glXGetProcAddressARB */
 } OpenGL_extension;