rtmpdump: fix CFLAGS/LDFLAGS

The rtmpdump makefile looks at XCFLAGS/XLDFLAGS for additional flags,
so use those instead of completely overriding the flags.

Fixes http://autobuild.buildroot.net/results/190/1902a6613e5fc9656842fbcbd5731ac871d7ef59/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>