avutil/libm: use FFMIN instead of fmin()

MSVC apparently doesnt support fmin() either

Suggested/Found-by: ubitux, Daemon404, nevcairiel
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/libm.h

@@ -84,7 +84,7 @@ static av_always_inline float cbrtf(float x)
 
 #if !HAVE_FMINF
 #undef fminf
-#define fminf(x, y) ((float)fmin(x,y))
+#define fminf(x, y) (FFMIN(x,y))
 #endif
 
 #if !HAVE_ISINF