Don't include stdbool.h when included from C++

As noted by Thiago Macieira, we shouldn't include stdbool.h when were
included from a C++-file such as main.cpp

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurfacestartup.h

@@ -4,10 +4,11 @@
 #include "dive.h"
 #include "divelist.h"
 #include "libdivecomputer.h"
-#include <stdbool.h>
 
 #ifdef __cplusplus
 extern "C" {
+#else
+#include <stdbool.h>
 #endif
 
 extern bool imported;