meson: don't build with USE_ELF_TLS on OpenBSD

OpenBSD does not have TLS

Fixes: a47c525f328 ("meson: build glx")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5630>
diff --git a/meson.build b/meson.build
index 85a724f..edb49e0 100644
--- a/meson.build
+++ b/meson.build
@@ -428,7 +428,7 @@
 
 # Android uses emutls for versions <= P/28. For USE_ELF_TLS we need ELF TLS.
 use_elf_tls = false
-if not ['windows', 'freebsd'].contains(host_machine.system()) and (not with_platform_android or get_option('platform-sdk-version') >= 29)
+if not ['windows', 'freebsd', 'openbsd'].contains(host_machine.system()) and (not with_platform_android or get_option('platform-sdk-version') >= 29)
   pre_args += '-DUSE_ELF_TLS'
   use_elf_tls = true
 endif