Fix backtrace for android

src/unix/notbsd/linux/mod.rs

@@ -259,6 +259,8 @@ cfg_if! {
                           newlen: size_t)
                           -> ::c_int;
             pub fn ioctl(fd: ::c_int, request: ::c_ulong, ...) -> ::c_int;
+            pub fn backtrace(buf: *mut *mut ::c_void,
+                             sz: ::c_int) -> ::c_int;
         }
     }
 }

src/unix/notbsd/mod.rs

@@ -318,9 +318,6 @@ extern {
     pub fn memalign(align: ::size_t, size: ::size_t) -> *mut ::c_void;
     pub fn setgroups(ngroups: ::size_t,
                      ptr: *const ::gid_t) -> ::c_int;
-    #[cfg(not(target_env = "musl"))]
-    pub fn backtrace(buf: *mut *mut ::c_void,
-                     sz: ::c_int) -> ::c_int;
 }
 
 cfg_if! {