x86/dsputil: fix compilation when h263 decoder/encoder are disabled.

The symbol "ff_h263_loop_filter_strength" is defined in h263.c, but
the h263 loopfilter functions (in the .asm file) are not optimized
out (even though their function pointers are never assigned).

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
