Fix compiling on VS2019

In VS2019, although the math functions are intrinsic, they need to be
declared by the math.h header. So the header must be included in the
ifndef block.

src/ExtMath.c

@@ -5,6 +5,7 @@
 #include <stdlib.h>
 
 #ifndef __GNUC__
+#include <math.h>
 float Math_AbsF(float x)  { return fabsf(x); /* MSVC intrinsic */ }
 float Math_SqrtF(float x) { return sqrtf(x); /* MSVC intrinsic */ }
 #endif