vrend: fix autotools build error

With the prior patch and without this one, I get

../src/.libs/libvrend.a(vrend_renderer.o): In function `vrend_format_can_scanout':
src/vrend_renderer.c:703: undefined reference to `gbm'
src/vrend_renderer.c:703: undefined reference to `gbm
src/vrend_renderer.c:706: undefined reference to `gbm'
../src/.libs/libvrend.a(vrend_renderer.o): In function `vrend_resource_gbm_init':
/virglrenderer/src/vrend_renderer.c:6337: undefined reference to `gbm'
/virglrenderer/src/vrend_renderer.c:6337: undefined reference to `gbm'

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

src/virglrenderer.c

@@ -41,7 +41,6 @@
 #ifdef HAVE_EPOXY_EGL_H
 #include "virgl_gbm.h"
 #include "virgl_egl.h"
-struct virgl_gbm *gbm = NULL;
 #endif
 
 #ifdef HAVE_EPOXY_GLX_H

src/vrend_renderer.c

@@ -10148,6 +10148,7 @@ void vrend_print_context_name(struct vrend_context *ctx)
 
 #ifdef HAVE_EPOXY_EGL_H
 struct virgl_egl *egl = NULL;
+struct virgl_gbm *gbm = NULL;
 #endif
 
 int virgl_has_gl_colorspace(void)

src/vrend_renderer.h

@@ -68,6 +68,7 @@ enum {
 extern int use_context;
 #ifdef HAVE_EPOXY_EGL_H
 extern struct virgl_egl *egl;
+extern struct virgl_gbm *gbm;
 #endif