meson: Ensure we always build Xext/hashtable.c for glx

Seems that while glxvnd relies on some of the hashtable functions in
Xext, we only build hashtable support for Xext if we're also building
the res extension. This leads to some errors if you try to build glx
without res enabled:

glx/liblibglxvnd.a(vndcmds.c.o): In function `LookupVendorPrivDispatch':
/home/lyudess/Projects/xserver/glx/vndcmds.c:65: undefined reference to `ht_find'
/home/lyudess/Projects/xserver/glx/vndcmds.c:67: undefined reference to `ht_add'
glx/liblibglxvnd.a(vndcmds.c.o): In function `GlxDispatchInit':
/home/lyudess/Projects/xserver/glx/vndcmds.c:405: undefined reference to `ht_generic_compare'
/home/lyudess/Projects/xserver/glx/vndcmds.c:405: undefined reference to `ht_generic_hash'
/home/lyudess/Projects/xserver/glx/vndcmds.c:405: undefined reference to `ht_create'
glx/liblibglxvnd.a(vndcmds.c.o): In function `GlxDispatchReset':
/home/lyudess/Projects/xserver/glx/vndcmds.c:468: undefined reference to `ht_destroy'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

So, make sure that hashtable.c gets both for both glx and res

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>