Add winpthreads.h -- not sure if this will actually work in MSVC compiliaton for true threading support.

But at least it let's it compile and satisfied the pthread calls.
For standalone apps threading is not currently used although pthread mutex calls are used.
https://github.com/Hamlib/Hamlib/issues/1107#

include/hamlib/rig.h

@@ -37,7 +37,11 @@
 #include <string.h>
 #include <inttypes.h>
 #include <time.h>
+#if defined(_WIN32)
+#include <winpthreads.h>
+#else
 #include <pthread.h>
+#endif
 
 /* Rig list is in a separate file so as not to mess up w/ this one */
 #include <hamlib/riglist.h>