winpthreads: fix `make dist`

libwinpthread_la_SOURCES lists header files which are no longer present
in the source tree. This causes `make dist` to fail.

Explicitly add `build-aux/windres-rc` to EXTRA_DIST.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>