loader: Add prototype for gfx_interp_md

This function will be used to draw in the graphics bindings when the
loader is compiled with graphics (gfx) support. Provide definitions
for lua and the simple interpreter. 4th support is forthcoming.

Sponsored by:		Netflix
Reviewed by:		kevans, jhb
Differential Revision:	https://reviews.freebsd.org/D43903

stand/common/gfx_fb.h

@@ -281,6 +281,8 @@ void term_image_display(teken_gfx_t *, const teken_rect_t *);
 
 void reset_font_flags(void);
 
+void gfx_interp_md(void);
+
 #ifdef __cplusplus
 }
 #endif

stand/common/interp_simple.c

@@ -198,3 +198,11 @@ interp_include(const char *filename)
 	}
 	return(res);
 }
+
+/*
+ * There's no graphics commands for the simple interpreter.
+ */
+void
+gfx_interp_md(void)
+{
+}

stand/liblua/gfx_utils.c

@@ -240,3 +240,8 @@ luaopen_gfx(lua_State *L)
 	luaL_newlib(L, gfxlib);
 	return 1;
 }
+
+void
+gfx_interp_md(void)
+{
+}