Mark as deprecated some macros which were presumably meant to be private API and accidentally exposed in the public h...

Original commit message from CVS:
* docs/libs/gst-plugins-base-libs-sections.txt:
* gst-libs/gst/rtp/gstbasertpdepayload.c:
* gst-libs/gst/rtp/gstbasertpdepayload.h:
Mark as deprecated some macros which were presumably meant to be
private API and accidentally exposed in the public header file.
Also actually _init() lock (only works at the moment because the
struct is zeroed out when created and the initial values in the
mutex struct are zeroes too). (#459585)