OsConstants: fix mac build

  host C++: libjavacore_32 <= libcore/luni/src/main/native/cbigint.cpp
  libcore/luni/src/main/native/android_system_OsConstants.cpp:385:45: error: use of undeclared identifier 'PR_GET_DUMPABLE'
      initConstant(env, c, "PR_GET_DUMPABLE", PR_GET_DUMPABLE);
                                              ^
  host C++: libjavacore_32 <= libcore/luni/src/main/native/java_io_File.cpp
  libcore/luni/src/main/native/android_system_OsConstants.cpp:386:45: error: use of undeclared identifier 'PR_SET_DUMPABLE'
      initConstant(env, c, "PR_SET_DUMPABLE", PR_SET_DUMPABLE);
                                              ^
  2 errors generated.

Change-Id: I2086795f9dcefdbe264c8c53b0df3149001c66ae