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>
diff --git a/libbacktrace/BacktraceImpl.h b/libbacktrace/BacktraceImpl.h
index 8ed64b3..d34ad05 100755
--- a/libbacktrace/BacktraceImpl.h
+++ b/libbacktrace/BacktraceImpl.h
@@ -21,7 +21,6 @@
 #include <backtrace/BacktraceMap.h>
 
 #include <sys/types.h>
-#include <ucontext.h>
 
 class BacktraceImpl {
 public:
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>
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>