Makefile: Fix wrong linker order

Compiles fine on both GCC 4.5 and 4.7 now.

Without this, fails with errors like:
/home/jon/btstats/statplug/reqsize.c:78: undefined reference to `g_free'
statplug/seek.o: In function `seek_init':
/home/jon/btstats/statplug/seek.c:73: undefined reference to `g_malloc0_n'
statplug/seek.o: In function `seek_destroy':
/home/jon/btstats/statplug/seek.c:86: undefined reference to `g_free'
statplug/seek.o: In function `seek_ops_init':
/home/jon/btstats/statplug/seek.c:94: undefined reference to `g_tree_insert'
trace_reader/trace.o: In function `find_input_traces':
/home/jon/btstats/trace_reader/trace.c:129: undefined reference to `g_malloc_n'
/home/jon/btstats/trace_reader/trace.c:130: undefined reference to `g_slist_prepend'
/home/jon/btstats/trace_reader/trace.c:143: undefined reference to `g_slist_length'
/home/jon/btstats/trace_reader/trace.c:146: undefined reference to `g_slist_foreach'
Fix warnings about unused-but-set parameters

Recent GCCs will complain about this. To allow functions to have correct
API but not use all parameters, use -Wno-unused-parameters
Enable -Werror