access: rtp: remove explicit _DEPENDENCIES
The _DEPENDENCIES variable is automatically filled when adding internal convenience libraries to _LIBADD and is not supposed to be overriden.
With LIBADD only, the dependency leads to the following makefile output:
am__DEPENDENCIES_25 = libvlc_srtp.la \
$(am__DEPENDENCIES_1)
librtp_plugin_la_DEPENDENCIES = libvlc_rtp.la $(am__DEPENDENCIES_1) \
$(am__DEPENDENCIES_25)