diagnostics: don't crash when libtextstyle is installed

Reported by neok m4700.
https://lists.gnu.org/archive/html/bison-patches/2019-05/msg00025.html
https://github.com/akimd/bison/pull/11

* src/complain.c (complain_init_color): style_file_prepare _needs_ a
string as second argument.

src/complain.c

@@ -255,7 +255,7 @@ complain_init_color (void)
       || color_mode == color_html
       || (color_mode == color_tty && isatty (STDERR_FILENO)))
     {
-      style_file_prepare ("BISON_STYLE", NULL, pkgdatadir (),
+      style_file_prepare ("BISON_STYLE", "BISON_STYLEDIR", pkgdatadir (),
                           "bison-default.css");
       /* As a fallback, use the default in the current directory.  */
       struct stat statbuf;