egl: hide entrypoints that shouldn't be exported when using glvnd

From GLVND author:
> From a functional standpoint, exporting additional symbols doesn't
> really matter, since libglvnd will load the vendor libraries with
> RTLD_LOCAL.

Suggested-by: Kyle Brenneman <kbrenneman@nvidia.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Kyle Brenneman <kbrenneman@nvidia.com>

src/egl/main/eglapi.c

@@ -83,6 +83,12 @@
  */
 
 
+#ifdef USE_LIBGLVND
+#define EGLAPI
+#undef PUBLIC
+#define PUBLIC
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>