glocalfileinfo.c compile failure when utimensat() not available (= MacOS)

The commit b33ef610 added the possibility of using the utimensat() function gated on a HAVE_UTIMENSAT define, but didn't put an #ifdef around the actual invocation of the function! On MacOS this leads to:

../gio/glocalfileinfo.c:2803:9: error: implicit declaration of function 'utimensat' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  res = utimensat (AT_FDCWD, filename, times_n, 0);