Only test-compile w32pthreads.h if W32THREADS are available.

This fixes 'make checkheaders' on non-W32THREADS systems.

libavcodec/Makefile

@@ -670,6 +670,7 @@ SKIPHEADERS-$(CONFIG_LIBSCHROEDINGER)  += libschroedinger.h
 SKIPHEADERS-$(CONFIG_VAAPI)            += vaapi_internal.h
 SKIPHEADERS-$(CONFIG_VDPAU)            += vdpau.h
 SKIPHEADERS-$(CONFIG_XVMC)             += xvmc.h
+SKIPHEADERS-$(HAVE_W32THREADS)         += w32pthreads.h
 
 EXAMPLES = api