Remove #include <syscall.h> to avoid mac os build error

Issue was found by hugohn in
#5 (comment)

Author: ryanofsky

src/mp/util.cpp

@@ -13,9 +13,12 @@
 #include <sys/types.h>
 #include <sys/un.h>
 #include <sys/wait.h>
-#include <syscall.h>
 #include <unistd.h>
 
+#if __linux__
+#include <syscall.h>
+#endif
+
 namespace mp {
 namespace {