Fix a buggy test case.

It was calling fabs() without having included <math.h>, causing fabs() to be treated as returning an int rather than a floating-point value. This misinterpretation of the return value could cause test failures.

Tests/Spec.Conform/SPC4636.1.CC

@@ -2,6 +2,8 @@
 /* arrays defined in the main source file.  Part of Special Conformance  */
 /* Test 4.6.3.6                                                          */
 
+#include <math.h>
+#include <stdio.h>
 
 int ExternTest (void)
   {