mpegaudiodec: replace pow(2.0,...) by exp2()

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

libavcodec/mpegaudiodec.c

@@ -26,6 +26,7 @@
 
 #include "libavutil/audioconvert.h"
 #include "libavutil/avassert.h"
+#include "libavutil/libm.h"
 #include "avcodec.h"
 #include "get_bits.h"
 #include "mathops.h"
@@ -394,7 +395,7 @@ static av_cold void decode_init_static(void)
 
         for (j = 0; j < 2; j++) {
             e = -(j + 1) * ((i + 1) >> 1);
-            f = pow(2.0, e / 4.0);
+            f = exp2(e / 4.0);
             k = i & 1;
             is_table_lsf[j][k ^ 1][i] = FIXR(f);
             is_table_lsf[j][k    ][i] = FIXR(1.0);