Only include ucontext.h when necessary.

Change-Id: I29014f6c6b82e0a9797e5d76147b3c4e3d82a7f5
diff --git a/libbacktrace/BacktraceImpl.cpp b/libbacktrace/BacktraceImpl.cpp
index d7ac0b2..405b042 100644
--- a/libbacktrace/BacktraceImpl.cpp
+++ b/libbacktrace/BacktraceImpl.cpp
@@ -19,6 +19,7 @@
 #include <string.h>
 #include <sys/ptrace.h>
 #include <sys/types.h>
+#include <ucontext.h>
 #include <unistd.h>
 
 #include <string>