ratecontrol.c: fixed exp2f on BeOS so rate control works properly

git-svn-id: svn://svn.videolan.org/x264/trunk@148 df754926-b1dd-0310-bc7b-ec298dee348c

encoder/ratecontrol.c

@@ -40,7 +40,7 @@
 #ifdef SYS_MACOSX
 #define exp2f(x) ( (float) exp2( (x) ) )
 #endif
-#ifdef SYS_FREEBSD
+#if defined(SYS_FREEBSD) || defined(SYS_BEOS)
 #define exp2f(x) powf( 2, (x) )
 #endif
 #ifdef _MSC_VER