Export gf_isom_set_pixel_aspect_ratio.
media-video/x264-encoder needs it and this funcion used to be exported.

diff --git a/src/isomedia/isom_write.c b/src/isomedia/isom_write.c
index e935acc..9841d85 100644
--- a/src/isomedia/isom_write.c
+++ b/src/isomedia/isom_write.c
@@ -1270,6 +1270,7 @@ GF_Err gf_isom_set_visual_info(GF_ISOFile *movie, u32 trackNumber, u32 StreamDes
 	}
 }
 
+GF_EXPORT
 GF_Err gf_isom_set_pixel_aspect_ratio(GF_ISOFile *movie, u32 trackNumber, u32 StreamDescriptionIndex, u32 hSpacing, u32 vSpacing)
 {
 	GF_Err e;