lavu: Makefile: skip atomic.c if native atomics are available

This prevents a linker warning and skips a useless compilation
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/Makefile

@@ -78,7 +78,6 @@ BUILT_HEADERS = avconfig.h                                              \
 
 OBJS = adler32.o                                                        \
        aes.o                                                            \
-       atomic.o                                                         \
        audio_fifo.o                                                     \
        avstring.o                                                       \
        base64.o                                                         \
@@ -137,6 +136,8 @@ OBJS = adler32.o                                                        \
        xga_font_data.o                                                  \
        xtea.o                                                           \
 
+OBJS-$(!HAVE_ATOMICS_NATIVE)            += atomic.o                     \
+
 OBJS-$(CONFIG_LZO)                      += lzo.o
 OBJS-$(CONFIG_OPENCL)                   += opencl.o opencl_internal.o