Only include ucontext.h when necessary.

Change-Id: I29014f6c6b82e0a9797e5d76147b3c4e3d82a7f5
diff --git a/libbacktrace/UnwindCurrent.cpp b/libbacktrace/UnwindCurrent.cpp
index e5f7c3d..b176aaf 100755
--- a/libbacktrace/UnwindCurrent.cpp
+++ b/libbacktrace/UnwindCurrent.cpp
@@ -14,8 +14,8 @@
  * limitations under the License.
  */
 
-#include <sys/ucontext.h>
 #include <sys/types.h>
+#include <ucontext.h>
 
 #include <backtrace/Backtrace.h>
 #include <backtrace/BacktraceMap.h>