Fix sigset definition. (#135)

Fix the definition of sigset, not to pull in definitions more than once.

Pal/src/host/FreeBSD/include/sigset.h

@@ -24,6 +24,8 @@
 #define _SIGSET_H_types 1
 #define _SIGSET_H_fns   1
 
+#define ____sigset_t_defined
+
 typedef int __sig_atomic_t;
 
 /* A `sigset_t' has a bit for each signal.  */

Pal/src/host/Linux-SGX/include/sigset.h

@@ -24,6 +24,8 @@
 #define _SIGSET_H_types 1
 #define _SIGSET_H_fns   1
 
+#define ____sigset_t_defined
+
 typedef int __sig_atomic_t;
 
 /* A `sigset_t' has a bit for each signal.  */

Pal/src/host/Linux/include/sigset.h

@@ -24,6 +24,8 @@
 #define _SIGSET_H_types 1
 #define _SIGSET_H_fns   1
 
+#define ____sigset_t_defined
+
 typedef int __sig_atomic_t;
 
 /* A `sigset_t' has a bit for each signal.  */