src: Disable use of getpass() when it is not available

For example, getpass is not available on Android platform.

Change-Id: I4c3039a3fcf188fd9343d625a551d268a4a20a93
diff --git a/src/security/sec-tpm.cpp b/src/security/sec-tpm.cpp
index 375aa1b..eb107b1 100644
--- a/src/security/sec-tpm.cpp
+++ b/src/security/sec-tpm.cpp
@@ -26,6 +26,7 @@
 #include "../encoding/oid.hpp"
 #include "../encoding/buffer-stream.hpp"
 #include "cryptopp.hpp"
+#include <unistd.h>
 
 namespace ndn {
 
@@ -362,6 +363,7 @@
 {
   bool isInitialized = false;
 
+#ifndef NDN_CXX_HAVE_GETPASS
   char* pw0 = 0;
 
   pw0 = getpass(prompt.c_str());
@@ -389,6 +391,8 @@
   if (password.empty())
     return false;
 
+#endif // NDN_CXX_HAVE_GETPASS
+
   return isInitialized;
 }