gst-libs/gst/video/gstvideoaggregator.c: fix build with gcc 4.8

gst-libs/gst/video/gstvideoaggregator.c: fix build with gcc 4.8

Fix the following build failure with gcc 4.8 which has been added with
https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/commit/d268c193ad39fb970351ed62898be806ebd0a71e:

../gst-libs/gst/video/gstvideoaggregator.c: In function 'gst_video_aggregator_init':
../gst-libs/gst/video/gstvideoaggregator.c:2762:3: error: 'for' loop initial declarations are only allowed in C99 mode
   for (gint i = 0; i < gst_caps_get_size (src_template); i++) {
   ^

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/974>