dmx: Mark glxIsExtensionSupported as a const char *

Gets rid of 9 instances of gcc 4.8 warning:

glxcmds.c: In function ‘CreateContext’:
glxcmds.c:378:13: warning: passing argument 1 of ‘glxIsExtensionSupported’
 discards ‘const’ qualifier from pointer target type [enabled by default]
             else if (glxIsExtensionSupported("GLX_SGIX_fbconfig")) {
             ^
In file included from glxserver.h:49:0,
                 from glxcmds.c:41:
glxscreens.h:53:12: note: expected ‘char *’ but argument is of type
 ‘const char *’
 extern int glxIsExtensionSupported(char *ext);

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Rémi Cardona <remi@gentoo.org>
Reviewed-by: Keith Packard <keithp@keithp.com>

hw/dmx/glxProxy/glxscreens.c

@@ -335,7 +335,7 @@ __glXGetServerString(unsigned int name)
 }
 
 int
-glxIsExtensionSupported(char *ext)
+glxIsExtensionSupported(const char *ext)
 {
     return (strstr(ExtensionsString, ext) != NULL);
 }

hw/dmx/glxProxy/glxscreens.h

@@ -50,6 +50,6 @@ extern void __glXScreenReset(void);
 
 extern char *__glXGetServerString(unsigned int name);
 
-extern int glxIsExtensionSupported(char *ext);
+extern int glxIsExtensionSupported(const char *ext);
 
 #endif                          /* !__GLX_screens_h__ */