Use internal extern "C" { ... } for C++

- removes warnings compiling swig engine
...
[ 10%] Generating swig-engine.cpp
.../libgnucash/engine/engine-helpers.h:31: Warning 313: Unrecognized extern type "C++".
.../libgnucash/engine/gnc-date.h:83: Warning 313: Unrecognized extern type "C++".
.../libgnucash/engine/qofquery.h:90: Warning 302: Identifier 'QofQuery' redefined (ignored),
.../libgnucash/engine/gnc-option.hpp:55: Warning 302: previous definition of 'QofQuery'.
.../libgnucash/engine/gnc-commodity.h:56: Warning 313: Unrecognized extern type "C++".
.../libgnucash/engine/gncBusiness.h:40: Warning 313: Unrecognized extern type "C++".
.../libgnucash/engine/gncEntry.h:37: Warning 313: Unrecognized extern type "C++".

libgnucash/engine/SplitP.h

@@ -52,6 +52,10 @@
 #include "qof.h"
 
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** STRUCTS *********************************************************/
 /* A "split" is more commonly referred to as an "entry" in a "transaction".
  */
@@ -232,6 +236,10 @@ typedef struct
 
 SplitTestFunctions* _utest_split_fill_functions (void);
 
+#ifdef __cplusplus
+}
+#endif
+
 
 /*@}*/