Fix mac build.

Forward declare ucontext_t so that it's not necessary for ucontext.h
to be included in the global header.

(cherry picked from commit b138037b26)

Change-Id: I0ffb5bf7c53da8467dd8770880ff412a5158d639

include/backtrace/Backtrace.h

@@ -19,7 +19,6 @@
 
 #include <inttypes.h>
 #include <stdint.h>
-#include <ucontext.h>
 
 #include <string>
 #include <vector>
@@ -48,6 +47,9 @@ struct backtrace_frame_data_t {
 // Forward declarations.
 class BacktraceImpl;
 
+struct ucontext;
+typedef ucontext ucontext_t;
+
 class Backtrace {
 public:
   // Create the correct Backtrace object based on what is to be unwound.