Fix test for is_desktop_gl.

We're looking for "return true if no ES", and strcmp returns != 0 if
not match (the ES string).

src/dispatch_common.c

@@ -108,7 +108,7 @@ epoxy_is_desktop_gl(void)
     const char *version = (const char *)glGetString(GL_VERSION);
 
     printf("VERSION %s\n", version);
-    return !strncmp(es_prefix, version, strlen(es_prefix));
+    return strncmp(es_prefix, version, strlen(es_prefix));
 }
 
 PUBLIC int