aac: workaround for compilation on cygwin

On cygwin, math.h needs to be included before float.h because of a bug
in the system headers. Including libavutil/libm.h first works around
this issue.

Longer discussion of the topic:
http://thread.gmane.org/gmane.comp.video.ffmpeg.devel/128582

libavcodec/aaccoder.c

@@ -30,13 +30,14 @@
  * add sane pulse detection
  ***********************************/
 
+#include "libavutil/libm.h" // brought forward to work around cygwin header breakage
+
 #include <float.h>
 #include "avcodec.h"
 #include "put_bits.h"
 #include "aac.h"
 #include "aacenc.h"
 #include "aactab.h"
-#include "libavutil/libm.h"
 
 /** bits needed to code codebook run value for long windows */
 static const uint8_t run_value_bits_long[64] = {