gst-fsl-plugin: add fsl-mm-mp3enc and fsl-mm-flv dependences

gst-fsl-plugin: add fsl-mm-mp3enc and fsl-mm-flv dependences

For MX5 machines, gst-fsl-plugin has build-time dependences on the
pre-built fsl-mm-mp3enc and fsl-mm-flv codec libraries.

If the mp3enc library is not in the sysroot, the 'libmfw_gst_mp3enc.so'
is not built.

If the fsl-mm-flv is not in the sysroot, the configure process warns
about:

"Unable to determine soname of lib_flv_parser_arm9_elinux.3.0.so library"

and the resulting 'aiur_registry.arm11.cf' gets a wrong entry for
flv_parser library.

Signed-off-by: Javier Viguera <javier.viguera@digi.com>
Tested-by: Daiane Angolini <daiane.angolini@freescale.com>