Use flexible array member instead of fake size.

The _XimCacheStruct structure is followed in memory by two strings containing
fname and encoding. The memory was accessed using the last member of the
structure `char fname[1]`. That is a lie, prohibits us from using sizeof and
confuses checkers. Lets declare it properly as a flexible array, so compilers
don't complain about writing past that array. As bonus we can replace the
XOffsetOf with regular sizeof.

Fixes GCC8 error:
  In function 'strcpy',
      inlined from '_XimWriteCachedDefaultTree' at imLcIm.c:479:5,
      inlined from '_XimCreateDefaultTree' at imLcIm.c:616:2,
      inlined from '_XimLocalOpenIM' at imLcIm.c:700:5:
  /usr/include/bits/string_fortified.h:90:10: error: '__builtin_strcpy'
  forming offset 2 is out of the bounds [0, 1] [-Werror=array-bounds]
     return __builtin___strcpy_chk (__dest, __src, __bos (__dest));

Caused by this line seemingly writing past the fname[1] array:
  imLcIm.c:479:  strcpy (m->fname+strlen(name)+1, encoding);

Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>