make ftfont_driver static.

Author: Yuuki Harano

src/font.h

@@ -897,7 +897,6 @@ extern void ftfont_filter_properties (Lisp_Object, Lisp_Object);
 extern void ftfont_text_extents (struct font *, unsigned *, int,
 				 struct font_metrics *);
 extern void syms_of_ftfont (void);
-extern struct font_driver const ftfont_driver;
 #endif	/* HAVE_FREETYPE */
 #ifdef HAVE_X_WINDOWS
 extern struct font_driver const xfont_driver;

src/ftfont.c

@@ -47,6 +47,8 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include "ftfont.h"
 #include "pdumper.h"
 
+static struct font_driver const ftfont_driver;
+
 /* Flag to tell if FcInit is already called or not.  */
 static bool fc_initialized;