winpthreads: do not expose IN_WINPTHREAD in public header files

When building winpthreads, define WINPTHREAD_API in config.h
instead of public pthread_compat.h.

Definition of WINPTHREAD_API in pthread_compat.h will only be used
by client code.

Note that IN_WINPTHREAD is still defined.
It is checked for in mingw-w64's time.h.

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