bsd_signal.3: Fix the wrong version of _POSIX_C_SOURCE

bsd_signal.3: Fix the wrong version of _POSIX_C_SOURCE

According to the latest glibc, the bsd_signal() function is just
declared when POSIX.1-2008 (or newer) instead of POSIX.1-2001 is
not set since glibc v2.26.

Please see the following code from signal/signal.h:
-----------------------------------------------------------------
/* The X/Open definition of `signal' conflicts with the BSD version.
   So they defined another function `bsd_signal'.  */
extern __sighandler_t bsd_signal (int __sig, __sighandler_t __handler)
     __THROW;
-----------------------------------------------------------------

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>