gthr-posix.h (__gthread_active_ptr): Cast function pointer to target type to allow ISO C++ compiler to accept it.

	* gthr-posix.h (__gthread_active_ptr): Cast function pointer
	to target type to allow ISO C++ compiler to accept it.
	* gthr-dce.h (__gthread_active_ptr): Likewise.
	* gthr-solaris.h (__gthread_active_ptr): Likewise.

Co-Authored-By: John David Anglin <dave@hiauly1.hia.nrc.ca>

From-SVN: r42725