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>

libavcodec/x86/dsputil.asm

@@ -653,7 +653,7 @@ BSWAP32_BUF
 INIT_XMM ssse3
 BSWAP32_BUF
 
-
+%if CONFIG_H263_DECODER || CONFIG_H263_ENCODER
 %macro H263_LOOP_FILTER 5
     pxor         m7, m7
     mova         m0, [%1]
@@ -813,3 +813,4 @@ cglobal h263_h_loop_filter, 3,5,0,32
     punpckhdq    m6, m6
     movd    [r4+r3], m6
     RET
+%endif ; CONFIG_H263_DECODER || CONFIG_H263_ENCODER