Fix build with GCC 8: strncpy ->strcpy

Fix build with GCC 8: strncpy ->strcpy

Recent gcc 8 trunk emits the warning below,

../../binutils-gdb/gdb/python/py-gdb-readline.c:79:15: error: ‘char* strncpy(char*, const char*, size_t)’ output truncated before terminating nul copying as many bytes from a string as its length [-Werror=stringop-truncation]
       strncpy (q, p, n);
       ~~~~~~~~^~~~~~~~~
../../binutils-gdb/gdb/python/py-gdb-readline.c:73:14: note: length computed here
   n = strlen (p);
       ~~~~~~~^~~

gdb:

2017-11-22  Yao Qi  <yao.qi@linaro.org>

	* python/py-gdb-readline.c (gdbpy_readline_wrapper): Use strcpy.