avutil/libm: fix fminf() emulation build failure due to undefined FFMIN

Found-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/libm.h

@@ -86,7 +86,9 @@ static av_always_inline float cbrtf(float x)
 #undef fminf
 static av_always_inline av_const float fminf(float x, float y)
 {
-    return FFMIN(x, y);
+    //Note, the NaN special case is needed for C spec compliance, it should be
+    //optimized away if the users compiler is configured to assume no NaN
+    return x > y ? y : (x == x ? x : y);
 }
 #endif