ui/gtk-egl: Fix build failure when libgbm is not available

ui/gtk-egl: Fix build failure when libgbm is not available

Since commit 4872a023a59 ("ui/gtk-egl: guest fb texture needs
to be regenerated when reinitializing egl") we get on Ubuntu
18.04.4 LTS and Debian Buster (oldstable):

  $ ../configure --enable-virglrenderer
  [...]
  ui/gtk-egl.c: In function 'gd_egl_refresh':
  ui/gtk-egl.c:159:13: error: implicit declaration of function 'egl_dmabuf_release_texture' [-Werror=implicit-function-declaration]
    159 |             egl_dmabuf_release_texture(vc->gfx.guest_fb.dmabuf);
        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~
  ui/gtk-egl.c:159:13: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
  ui/gtk-egl.c:159:13: error: nested extern declaration of 'egl_dmabuf_release_texture' [-Werror=nested-externs]

Fix by restricting the egl_dmabuf_release_texture() call to the
availability of the generic buffer management library (libgbm).

Fixes: 4872a023a593e6519b272a
Cc: Dongwon Kim <dongwon.kim@intel.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Reported-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211108083129.1262040-1-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>