Only include ucontext.h when necessary.

Change-Id: I29014f6c6b82e0a9797e5d76147b3c4e3d82a7f5
diff --git a/libbacktrace/UnwindPtrace.cpp b/libbacktrace/UnwindPtrace.cpp
index 63b4f16..7ba8775 100644
--- a/libbacktrace/UnwindPtrace.cpp
+++ b/libbacktrace/UnwindPtrace.cpp
@@ -19,6 +19,7 @@
 
 #include <sys/types.h>
 #include <string.h>
+#include <ucontext.h>
 
 #include <libunwind.h>
 #include <libunwind-ptrace.h>