rtmpdump: use TARGET_LDFLAGS instead of TARGET_CFLAGS for XLDFLAGS

The XLDFLAGS is for ld related flags so use TARGET_LDFLAGS instead of
TARGET_CFLAGS, also the TARGET_CFLAGS is already used for XCFLAGS.

This fix is also needed for supporting the per-package staging directory.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>