vo_svga: fix build after previous commit 6506d4ad

vo_svga.c used its static query_format() function before defining it,
but had no prototype before the use. Thus is depended on the otherwise
useless static declaration in a shared header, and broke after that
was removed. Fix by adding an advance declaration inside the file.

libvo/vo_svga.c

@@ -63,7 +63,7 @@ TODO:
 
 #include <assert.h>
 
-
+static int query_format(uint32_t format);
 static void draw_alpha(int x0, int y0, int w, int h, unsigned char *src,
                        unsigned char *srca, int stride);
 static uint32_t get_image(mp_image_t *mpi);