Add missing prototype in xsbuild test.

The plus1 function needs a prototype under C as well as C++.
Otherwise any decent compiler will throw a warning.

Author: craigberry

t/lib/MakeMaker/Test/Setup/XS.pm

@@ -263,7 +263,11 @@ $label2files{xsbuild} = +{
 
   'lib/XS/Other.xs' => <<EOF,
 #ifdef __cplusplus
-extern "C" { int plus1(int); }
+extern "C" {
+#endif
+int plus1(int);
+#ifdef __cplusplus
+}
 #endif
 $XS_OTHER
 int