ntdll/signal_x86_64: include <ucontext.h> for FreeBSD

[Austin English]

Helps with http://bugs.winehq.org/show_bug.cgi?id=22863

-- 
-Austin

diff --git a/dlls/ntdll/signal_x86_64.c b/dlls/ntdll/signal_x86_64.c
index ccea6df..154ae34 100644
--- a/dlls/ntdll/signal_x86_64.c
+++ b/dlls/ntdll/signal_x86_64.c
@@ -29,6 +29,10 @@
 #include <stdarg.h>
 #include <stdio.h>

+#ifdef HAVE_UCONTEXT_H
+# include <ucontext.h>
+#endif
+
 #ifdef HAVE_UNISTD_H
 # include <unistd.h>
 #endif