gst/: Include stdlib.h in some more places, makes things compile with uClibc and...

gst/: Include stdlib.h in some more places, makes things compile with uClibc and -Werror (#357592).

Original commit message from CVS:
Patch by: Peter Kjellerstedt  <pkj at axis com>
* gst/alpha/gstalpha.c:
* gst/rtp/gstrtpamrdepay.c:
* gst/rtsp/gstrtspsrc.c:
* gst/udp/gstudpsrc.c:
* gst/videomixer/videomixer.c:
Include stdlib.h in some more places, makes things compile
with uClibc and -Werror (#357592).