Include the public header in the implementation file.

In the public header files, all symbols are marked extern C. When using
a C compiler, there is usually no problem if the header isn't included
in the C file. But the msvc build system uses the C++ compiler (due to
the use of some C99 features not supported by the msvc C compiler).

src/hw_ostc_parser.c

@@ -22,6 +22,7 @@
 #include <stdlib.h>
 
 #include <libdivecomputer/hw_ostc.h>
+#include <libdivecomputer/hw_ostc3.h>
 #include "libdivecomputer/units.h"
 
 #include "context-private.h"