xserver: miext/sync/meson.build: compile misyncshm.c if xshmfence found

It is a patch that fixes xserver build if libdrm is too old.

Now misyncshm.c compilation depends on dri3, which is incorrect. If libdrm
is not recent enough, then dri3 is not built, the file misyncshm.c
is not compiled, and the function miSyncShmScreenInit() is unavailable.
It is called in glamor_sync.c if xshmfence is present, which causes
a compilation error. This patch makes misyncshm.c compile if xshmfence
is found.

Signed-off-by: Oleh Nykyforchyn <olen.nyk@gmail.com>

miext/sync/meson.build

@@ -10,7 +10,7 @@ hdrs_miext_sync = [
     'misyncstr.h',
 ]
 
-if build_dri3
+if xshmfence_dep.found()
     srcs_miext_sync += 'misyncshm.c'
 endif