Include <sys/cdefs.h> earlier to get the various visibility constants.

Previously, we were relying on <sys/_types.h> to include it implicitly.

lib/msun/src/math.h

@@ -17,6 +17,7 @@
 #ifndef _MATH_H_
 #define	_MATH_H_
 
+#include <sys/cdefs.h>
 #include <sys/_types.h>
 
 /*
@@ -150,8 +151,6 @@ struct exception {
 
 #endif /* __BSD_VISIBLE */
 
-#include <sys/cdefs.h>
-
 /*
  * Most of these functions have the side effect of setting errno, so they
  * are not declared as __pure2.  (XXX: this point needs to be revisited,