Fix compiler warnings from GCC 8.1

Fix compiler warnings from GCC 8.1

Fix these 2 warnings:

 litex/build/sim/core/libdylib.c:42:5: warning: 'strncpy' specified bound 2048 equals destination size
 [-Wstringop-truncation]
     strncpy(last_err, s, ERR_MAX_SIZE);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 In function 'set_last_error',

 litex/soc/software/libbase/exception.c:28:13: warning: function declaration isn't a prototype [-Wstrict-prototypes]
  static char emerg_getc()