mingw: Make sure sigset_t is defined

With MSys2, the sigset_t type is defined in sys/types.h, therefore we
need to #include said file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.h

@@ -1,5 +1,9 @@
 #include <stdint.h>
 #include <wchar.h>
+#include <sys/types.h>
+#ifndef _POSIX
+typedef _sigset_t sigset_t;
+#endif
 #include <winsock2.h>
 #include <ws2tcpip.h>
 /*