include: Ensure float.h is included for DBL_MAX.

I didn't actually hit the issue in practice, but just happen to notice
while looking at the code.

Reviewed-by: Brian Paul <brianp@vmware.com>

include/c99_math.h

@@ -71,6 +71,7 @@ roundf(float x)
 #endif
 
 #ifndef INFINITY
+#include <float.h> // DBL_MAX
 #define INFINITY (DBL_MAX + DBL_MAX)
 #endif